Plasma immersion ion implantation apparatus including a plasma source having low dissociation and low minimum plasma voltage

ABSTRACT

A method for ion planting a species into a surface layer of a workpiece in a chamber, the method includes placing the workpiece in a processing zone of the chamber bounded by a chamber side wall and a chamber ceiling facing said workpiece and between a pair of ports of the chamber near generally opposite sides to the processing zone and connected together by a conduit external of the chamber. The method further includes introducing into the chamber a process gas comprising the species to be implanted, and further generating from the process gas a plasma current and causing the plasma current to oscillate in a circulatory reentrant path comprising the conduit and the processing zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 10/164,327 filed Jun. 5, 2003 by Kenneth Collins et al.,entitled EXTERNALLY EXCITED TORROIDAL PLASMA SOURCE WITH MAGNETICCONTROL OF ION DISTRIBUTION and assigned to the present assignee.

BACKGROUND OF THE INVENTION

[0002] The present invention is related to semiconductor microelectroniccircuit fabrication, and particularly to ion implantation using plasmaimmersion.

[0003] The formation of semiconductor junctions on the surface of asemiconductor crystal (such as silicon wafer) is generally carried outby implantation of ions of either acceptor or donor impurity species(e.g., Boron or Arsenic) into the surface. Currently, ion implantationis efficiently carried out by ion beam accelerators. An ion beamaccelerator raster-scans a beam of donor or acceptor ions across thesemiconductor wafer surface. The implanted semiconductor wafer surfaceis then annealed at elevated temperatures in excess of 600° C. in orderto cause the implanted species to be substituted for silicon atomswithin the crystal lattice. This process is defined as dopantactivation. The depth of the implanted species below the surface, inconjunction with a subsequent anneal process, determines the junctiondepth, which is determined by the kinetic energy of the ion beam andsubsequent annealing thermal budget. The conductance of the implantedregion of the semiconductor is determined by the junction depth and thevolume concentration of the thermally activated implanted dopantspecies. The implanted dopant species concentration is controlled by therate at which the ion beam is scanned across the semiconductor surfaceand the beam current. The activated implanted dopant speciesconcentration is controlled by the above, and the subsequent annealprocess (temperature and time characteristics). For currentsemiconductor fabrication processes, in which semiconductor circuitfeature size is about 130 nm, ion beam accelerators are suitable for ionimplantation because the junction depth is fairly deep (over 330Angstroms) and the required dopant dose is fairly modest (about 2×10¹⁴to about 2×10¹⁵ ions/cm²). Such a modest dopant concentration isfulfilled by an ion beam accelerator with an implant operation lastingonly minutes. Because of the deep junction depth, the abruptness of thejunction need be no smaller than 6 nm/dec (i.e., nanometers per decadeof concentration). Therefore, ion energy distribution is not critical,and some ions may have a kinetic energy that carries them somewhatbeyond the desired junction depth without degrading the abruptnessbeyond the 4.1 nm/dec level. Therefore, techniques for enhancing the ionbeam flux that compromise ion energy distribution can be used. Thesetechniques include using an ion beam that has a few times the kineticenergy corresponding to the desired junction depth, or about 2 keV, (andtherefore several times the ion flux density), and then electricallydecelerating the ion beam down to the correct kinetic energy (e.g., 500eV) just before it impacts the semiconductor wafer surface.

[0004] The deceleration process is not precise and leaves a fraction ofimplanted particles (neutrals) above the correct energy level, which issometimes referred to as a high energy tail or energy contamination. Thehigh energy tail arises from the natural occurrence of neutrals in theion beam and the immunity of such neutrals from the electrostaticdeceleration process. Such neutrals therefore impact the wafer at theoriginal energy (e.g., 2 keV), so that they are implanted below thedesired junction depth, due to the high energy tail, causing a loss ofjunction abruptness. But this is not harmful because of the relativelyrelaxed requirement for junction abruptness (6 nm/dec). Moreover, rapidthermal annealing by halogen lamps, for example, tends to “wash out” theeffect of the high energy tail due to diffusion.

[0005] However, as semiconductor circuit feature size decreases withprogress in device speed, ion beam accelerators become less efficient.For example, at a feature size of 65 nm, the junction depth is onlyabout 170 Angstroms and the abruptness is much steeper, at 2.8 nm/dec.With such a shallow junction, the required dopant dose is greater (toavoid an increased resistance), or about 10¹⁵ to about 2×10¹⁶ ions/cm².In order to activate such higher dopant concentrations in the siliconcrystal, and in order to avoid increasing junction depth duringannealing, dynamic surface annealing is advantageously employed, inwhich the wafer surface (e.g., down to depth of order 1000 Angstroms) islaser-heated to near melting (e.g., 1300 deg. C.) for a period of ananosecond to tens of milliseconds. Dynamic surface annealing activatesa higher concentration of dopant and increases junction depth by lessthan 20 Å compared with rapid thermal annealing. (By comparison, rapidthermal annealing can add over 100 Å to the junction depth, which woulddouble the junction depth in some cases.) However, dynamic surfaceannealing does not reduce the high energy tail. Therefore, in order tostay within the more stringent junction abruptness requirement and inorder to avoid a high energy tail, the ion beam accelerator must beoperated in drift mode, in which the ions are accelerated up to but notbeyond the kinetic energy corresponding to the desired junction depth(e.g., only 500 eV), so that no ions will be implanted below the desireddepth, and no deceleration process is required. For example, a junctiondepth of 10-20 nm may translate to an ion beam energy of only 500 eV.Unfortunately, the lower ion energy in drift mode limits the ion beamflux (and current), so that the time required to reach the desired highdopant concentration can be as long as a half hour or one hour. Thisproblem arises particularly in shallow junction implant of light speciessuch as Boron, in which the beam voltage must be reduced to avoid highvelocity Boron ions being implanted below the desired junction depth.The problem arises basically because the space charge effects in the ionbeam produce repulsive forces between the ions in the beam in a radialdirection, generally, limiting the beam density and therefore the beamcurrent. Such effects become more important as the beam energy isreduced (as it must be for implanting the lighter elements such asBoron), resulting in lower beam currents and longer implant times. Suchlong implant times greatly limit productivity and increase productioncosts. For example, in order to avoid a decrease in wafer through-put,the number of ion beam implant machines must be increased. In thefuture, feature sizes will decrease further, down to 45 nm, so that suchproblems will worsen in proportion as the technology advances.

[0006] These problems pertain particularly to cases in which the speciesto be implanted has a low atomic weight (such as Boron), so that theacceleration voltage must be small, which translates into a small ionbeam flux and a long implant time. For higher atomic weight species(such as Arsenic), the acceleration voltages are much higher and the ionbeam flux is therefore sufficiently high to keep implantation times downto an acceptable level. One way of permitting an increased beamacceleration voltage for lighter implant species such as Boron, in orderto improve ion flux and reduce implant time, is to implant molecularions consisting of one Boron atom or more and another volatile speciessuch as Fluorine, Hydrogen, or other species. Examples of such molecularions are BF₂, B₁₀H₁₄. Thus, implanting BF₂ permits the use of a muchhigher beam energy and therefore a higher and more acceptable ion beamflux. However, while much of the implanted fluorine tends to diffuse outof the silicon crystal during annealing, a significant amount does not,leaving some crystal lattice sites that contain neither a semiconductoratom (Si) nor a dopant impurity atom (B), thus (for some applications)reducing the overall quality of the semiconductor material. Therefore,this technique is not desirable universally for all applications.

[0007] In summary, advances in technology dictate a more shallowjunction depth, a greater junction abruptness and a higher dopantconcentration in the semiconductor surface. Such advances in technology(where features size decreases to 65 nm and ultimately to 45 nm) renderion beam implantation of lighter dopants such as Boron impractical. Thisis because the traditional ion beam implanter provides too little ionbeam flux in such applications.

[0008] In order to find an ion source having much higher ion flux forlow atomic weight species such as Boron, the field has turned to an ionsource whose flux at a given implant depth is less affected by the spacecharge effect or (indirectly) atomic weight, namely a plasma ion source.Specifically, the semiconductor wafer is immersed in a plasma consistingof dopant ions (such as Boron ions). However, such plasma ion immersionimplantation has been plagued by various difficulties.

[0009] One type of plasma immersion ion implantation reactor employs apulsed D.C. voltage applied to a pedestal supporting the semiconductorwafer in a vacuum chamber filled with a dopant-containing gas such asBF3. The D.C. voltage creates a plasma discharge in the chamber in whichBoron ions and other ions dissociated from the BF3 ions are acceleratedinto the wafer surface. The D.C. voltage maintains the plasma bycreation of secondary electrons from collisions with the chambersurfaces or wafer surface. The rate at which such collisions producesecondary electrons depends upon the condition of the chamber surfaces.Accordingly, such a reactor is unacceptably sensitive to changes in thecondition of the chamber surfaces due, for example, to contamination ofthe chamber surfaces. As a result, such a plasma ion immersionimplantation reactor cannot maintain a target junction depth orabruptness, for example, and is plagued by contamination problems.

[0010] This type of reactor tends to produce a relatively low densityplasma and must be operated at relatively high chamber pressure in orderto maintain the plasma density. The high chamber pressure and the lowerplasma density dictate a thicker plasma sheath with more collisions inthe sheath that spread out ion energy distribution. This spreading canresult in a larger lateral junction distribution and may reduce junctionabruptness. Furthermore, the reactor is sensitive to conditions on thewafer backside because the plasma discharge depends upon ohmic contactbetween the wafer backside and the wafer support pedestal.

[0011] One problem inherent with D.C. voltage applied to the wafersupport is that its pulse width must be such that the dopant ions (e.g.,Boron) are accelerated across the plasma sheath near the wafer surfacewith sufficient energy to reach the desired junction depth below thesurface, while the pulse width must be limited to avoid (discharge) anycharge build-up on the wafer surface that would cause device damage(charging damage). The limited pulse width is problematic in that theperiodic decrease in ion energy can result in deposition on thesemiconductor surface rather than implantation, the depositionaccumulating in a new layer that can block implantation during the pulseon times. Another problem arises because ions must impact the wafersurface with at least a certain target energy in order to penetrate thesurface up to a desirable depth (the as-implanted junction depth) andbecome substitutional below the surface and up to the desired annealedjunction depth during the annealing process. Below this energy, they donot penetrate the surface up to the as-implanted junction depth and donot become substitutional at the desired junction depth upon annealing.Moreover, the ions below the target energy may simply be deposited onthe wafer surface, rather than being implanted, to produce a film thatcan impede implantation. Unfortunately, due to resistive and capacitivecharging effects (RC time constant) on dielectric films on the waferthat tend to accompany a D.C. discharge, the ions reach the targetenergy during only a fraction of each pulse period (e.g., during thefirst microsecond), so that there is an inherent inefficiency. Moreover,the resulting spread in ion energy reduces the abruptness of the P-Njunction. This problem cannot be solved by simply increasing the biasvoltage, since this would increase the junction depth beyond the desiredjunction depth.

[0012] Another type of plasma immersion ion implantation reactor employsinductive coupling to generate the plasma, in addition to the pulsedD.C. voltage on the wafer. This type of reactor reduces the problemsassociated with plasma maintenance from secondary electrons, but stillsuffers from the problems associated with pulsed D.C. voltages on thewafer discussed immediately above.

[0013] Another type of plasma ion immersion implantation reactor employsan RF voltage applied to the wafer support pedestal that both controlsion energy and maintains the plasma. As in the pulsed D.C. voltagediscussed above, the RF voltage on the wafer support creates a plasmadischarge in the chamber in which Boron ions and other ions dissociatedfrom the BF₃ ions are accelerated into the wafer surface. The RF voltagegenerates and maintains the plasma mainly by capacitively coupling RFenergy from the electrode across the sheath to electrons in the plasmajust above the sheath (low pressure case) or electrons in the bulkplasma volume (high pressure case). While such a reactor has reducedsensitivity to chamber surface conditions as compared to reactorsemploying a pulsed DC bias, it is still quite sensitive. Also, ionenergy and flux cannot be independently selected with a single RF powersource. Ion flux may still be unacceptably low for high throughputapplications with a single RF power source. Contamination due to wallsputtering or etching may also be high due to elevated plasma potential.

[0014] Another type of plasma ion immersion implantation reactor employsa microwave power applicator for generating the plasma. This reactor hasa microwave waveguide pointed axially downward to a magnetic fieldcentered about the axis. Electron cyclotron resonance (ECR) occurs in aparticular surface of the field to produce the plasma (for a microwavefrequency of 2.45 GHz, this surface is where the magnetic field is about875 gauss). The magnetic field is divergent, with a field gradientcreating a drift current towards the substrate being processed. Thisdrift current consists of both electrons (directly acted on by theinteraction of microwave induced electric field and divergent DCmagnetic field) and positively-charged ions (indirectly acted on by thedeficit in negative charge formed due to the out-flux of electrons) andcorresponding to a voltage of 10 to 100 eV. One problem is that themagnetic field gradient is non-uniform, so that the radial distributionof plasma ion energy is non-uniform, causing non-uniform junction depthsacross the wafer. Another problem is the relatively high ion energydirected at the wafer, limiting the degree to which junction depths canbe minimized. One way of addressing the non-uniformity issue is to placethe microwave ECR source far above the wafer. The problem with such anapproach is that the ion density and flux is at least proportionatelydecreased, thus reducing the productivity of the reactor. A relatedproblem is that, because the plasma ion density at the wafer surface isreduced by the increased source-to-wafer distance, the chamber pressuremust be reduced in order to reduce recombination losses. This rules outsome applications that would be advantageously carried out at highpressure (applications which benefit from wide angular ion energydistribution) such as conformal doping of polysilicon lines and threedimensional devices. Another way of addressing the non-uniformity issueis to place another magnet array between the source and the wafer, in aneffort to straighten the magnetic field. However, the additionalmagnetic field would increase magnetic flux at the wafer surface,increasing the risk of charge damage to semiconductor structures on thewafer.

[0015] In summary, plasma immersion ion implantation reactors havevarious limitations, depending upon the type of reactor: plasma reactorsin which a pulsed D.C. voltage is applied to the wafer pedestal are toosensitive to chamber conditions and are inefficient; and plasma reactorswith microwave ECR sources tend to produce non-uniform results. Thus,there is a need for a plasma immersion ion implantation reactor that isfree of the foregoing limitations.

SUMMARY OF THE INVENTION

[0016] A method for ion planting a species into a surface layer of aworkpiece in a chamber, the method includes placing the workpiece in aprocessing zone of the chamber bounded by a chamber side wall and achamber ceiling facing said workpiece and between a pair of ports of thechamber near generally opposite sides to the processing zone andconnected together by a conduit external of the chamber. The methodfurther includes introducing into the chamber a process gas comprisingthe species to be implanted, and further generating from the process gasa plasma current and causing the plasma current to oscillate in acirculatory reentrant path comprising the conduit and the processingzone.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 illustrates a first case that maintains an overheadtorroidal plasma current path.

[0018]FIG. 2 is a side view of a case corresponding to the case of FIG.1.

[0019]FIG. 3 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in wafer-to-ceiling gapdistance.

[0020]FIG. 4 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF bias power applied tothe workpiece.

[0021]FIG. 5 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF source power appliedto the coil antenna.

[0022]FIG. 6 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in reactor chamber pressure.

[0023]FIG. 7 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in partial pressure of aninert diluent gas such as Argon.

[0024]FIG. 8 is a graph illustrating the degree of dissociation ofprocess gas as a function of source power for an inductively coupledreactor and for a reactor according to an embodiment of the presentinvention.

[0025]FIG. 9 illustrates a variation of the case of FIG. 1.

[0026]FIGS. 10 and 11 illustrate a variation of the case of FIG. 1 inwhich a closed magnetic core is employed.

[0027]FIG. 12 illustrates another case of the invention in which atorroidal plasma current path passes beneath the reactor chamber.

[0028]FIG. 13 illustrates a variation of the case of FIG. 10 in whichplasma source power is applied to a coil wound around a distal portionthe closed magnetic core.

[0029]FIG. 14 illustrates a case that establishes two parallel torroidalplasma currents.

[0030]FIG. 15 illustrates a case that establishes a plurality ofindividually controlled parallel torroidal plasma currents.

[0031]FIG. 16 illustrates a variation of the case of FIG. 15 in whichthe parallel torroidal plasma currents enter and exit the plasma chamberthrough the vertical side wall rather than the ceiling.

[0032]FIG. 17A illustrates a case that maintains a pair of mutuallyorthogonal torroidal plasma currents across the surface of theworkpiece.

[0033]FIG. 17B illustrates the use of plural radial vanes in the case ofFIG. 17A.

[0034]FIGS. 18 and 19 illustrate an case of the invention in which thetorroidal plasma current is a broad belt that extends across a wide pathsuitable for processing large wafers.

[0035]FIG. 20 illustrates a variation of the case of FIG. 18 in which anexternal section of the torroidal plasma current path is constricted.

[0036]FIG. 21 illustrates a variation of the case of FIG. 18 employingcylindrical magnetic cores whose axial positions may be adjusted toadjust ion density distribution across the wafer surface.

[0037]FIG. 22 illustrates a variation of FIG. 21 in which a pair ofwindings are wound around a pair of groups of cylindrical magneticcores.

[0038]FIG. 23 illustrates a variation of FIG. 22 in which a singlecommon winding is wound around both groups of cores.

[0039]FIGS. 24 and 25 illustrate an case that maintains a pair ofmutually orthogonal torroidal plasma currents which are wide beltssuitable for processing large wafers.

[0040]FIG. 26 illustrates a variation of the case of FIG. 25 in whichmagnetic cores are employed to enhance inductive coupling.

[0041]FIG. 27 illustrates a modification of the case of FIG. 24 in whichthe orthogonal plasma belts enter and exit the reactor chamber throughthe vertical side wall rather than through the horizontal ceiling.

[0042]FIG. 28A illustrates an implementation of the case of FIG. 24which produces a rotating torroidal plasma current.

[0043]FIG. 28B illustrates a version of the case of FIG. 28A thatincludes magnetic cores.

[0044]FIG. 29 illustrates a preferred case of the invention in which acontinuous circular plenum is provided to enclose the torroidal plasmacurrent.

[0045]FIG. 30 is a top sectional view corresponding to FIG. 29.

[0046]FIGS. 31A and 31B are front and side sectional views correspondingto FIG. 30.

[0047]FIG. 32 illustrates a variation of the case 29 employing threeindependently driven RF coils underneath the continuous plenum facing at120-degree intervals.

[0048]FIG. 33 illustrates a variation of the case of FIG. 32 in whichthe three RF coils are driven at 120-degree phase to provide anazimuthally rotating plasma.

[0049]FIG. 34 illustrates a variation of the case of FIG. 33 in which RFdrive coils are wound around vertical external ends of respectivemagnetic cores whose opposite ends extend horizontally under the plenumat symmetrically distributed angles.

[0050]FIG. 35 is a version of the case of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the case of FIG. 20.

[0051]FIG. 36 is a version of the case of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

[0052]FIG. 37 is a case corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

[0053]FIG. 38 is a case corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

[0054]FIG. 39 is a case corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

[0055]FIG. 40 is a case corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

[0056]FIG. 41 is a case corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

[0057]FIG. 42 is a case corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

[0058]FIG. 43 is a case corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

[0059]FIG. 44 illustrates cases a reactor similar to that of FIG. 1 andhaving a magnetic pole piece for controlling plasma ion densityuniformity.

[0060]FIG. 45 illustrates a reactor like that of FIG. 44 in which themagnetic pole piece has a reduced diameter near the ceiling surface, andthe ceiling is a dual zone gas distribution plate.

[0061]FIGS. 46, 47 and 48 illustrate different shapes for the polepiece.

[0062]FIG. 49 illustrates one implementation of the gas distributionplate.

[0063]FIG. 50 is a detailed view of a gas injection orifice in FIG. 49.

[0064]FIG. 51 is a graph depicting the magnetic field that the magneticpole piece can generate.

[0065]FIG. 52 is a graph of the magnetic field magnitude as a functionof radius.

[0066]FIGS. 53 and 54 illustrate different ways of controlling processgas flow.

[0067]FIGS. 55A and 55B illustrate the use of a splitter in thetorroidal plasma path.

[0068]FIGS. 56A, 56B and 56C illustrate use of splitters where thetorroidal plasma current enters the chamber vertically.

[0069]FIGS. 57 and 58 illustrate different shapes for a splitter.

[0070]FIGS. 59A and 59B illustrate use of splitters where the torroidalplasma current enters the chamber radially.

[0071]FIGS. 60, 61, 62 and 63 illustrate the use of splitters where thetorroidal plasma current is introduced vertically at a corner of thechamber.

[0072]FIG. 64 illustrates how a splitter may extend only part of theprocess region height.

[0073]FIGS. 65A, 65B and 66 illustrate a splitter design adapted toincrease the effective radial path length of the torroidal plasmacurrent inside the chamber for a given chamber diameter.

[0074]FIG. 67 illustrates the use of MERIE magnets with the torroidalplasma current source of FIG. 1.

[0075]FIGS. 68 and 69 illustrate the use of fins to better confine thetorroidal plasma current to the processing region.

[0076]FIGS. 70, 71 and 72 illustrate an RF power applicator havingdistributed inductances.

[0077]FIG. 72 illustrates distributed inductances corresponding to theFIGS. 70, 71A and 71B.

[0078]FIG. 73 illustrates a circular arrangement of the distributedinductances of FIG. 72.

[0079]FIG. 74 illustrates distributed inductances and capacitances in anarrangement corresponding to that of FIGS. 71A and 71B.

[0080]FIGS. 75 and 76 are schematic diagrams illustrating different waysof inductively coupling RF power using the magnetic core of FIGS. 71Aand 71B.

[0081]FIG. 77 illustrates the use of an insulator layer to electricallyisolate the termination sections and torroidal tubes of FIG. 44.

[0082]FIG. 78 illustrates how the uniformity control magnet or magneticpole may be placed under the wafer support pedestal.

[0083]FIG. 79 depicts an inductively coupled plasma immersion ionimplantation reactor having an RF bias power applicator.

[0084]FIGS. 80A, 80B and 80C illustrate, respectively, an applied pulsedD.C. bias voltage, the corresponding sheath voltage behavior and anapplied RF bias voltage.

[0085]FIGS. 81A, 81B, 81C and 81D illustrate, respectively, an energydistribution of ion flux, a cycle of applied RF bias voltage, ionsaturation current as a function of D.C. bias voltage, and energydistribution of ion flux for different frequencies of RF bias voltage.

[0086]FIGS. 82A and 82B illustrate the temporal relationship between thepower output waveforms of the source power generator and the bias powergenerator in a push-pull mode.

[0087]FIGS. 82C and 82D illustrate the temporal relationship between thepower output waveforms of the source power generator and the bias powergenerator in an in-synchronism mode.

[0088]FIGS. 82E and 82F illustrate the temporal relationship between thepower output waveforms of the source power generator and the bias powergenerator in a symmetric mode.

[0089]FIGS. 82G and 82H illustrate the temporal relationship between thepower output waveforms of the source power generator and the bias powergenerator in a non-symmetric mode.

[0090]FIGS. 83A and 83B illustrate different versions of a capacitivelycoupled plasma immersion ion implantation reactor having an RF biaspower applicator.

[0091]FIG. 84 illustrates a plasma immersion ion implantation reactorhaving a reentrant torroidal path plasma source.

[0092]FIG. 85 illustrates a plasma immersion ion implantation reactorhaving a torroidal plasma source with two intersecting reentrant plasmapaths.

[0093]FIG. 86 illustrates an interior surface of the ceiling of thereactor of FIG. 85.

[0094]FIG. 87 illustrates a gas distribution panel of the reactor ofFIG. 85.

[0095]FIG. 88 is a partial view of the reactor of FIG. 85 modified toinclude a plasma control center electromagnet.

[0096]FIGS. 89A and 89B are side and top views, respectively, of aversion of the reactor of FIG. 88 having, in addition, a plasma controlouter electromagnet.

[0097]FIGS. 90A, 90B and 90C are cross-sectional side view of the outerelectromagnet of FIG. 89A with different gap distances of a bottom platefor regulating magnetic flux.

[0098]FIG. 91 illustrates an RF bias power coupling circuit in thereactor of FIG. 85.

[0099]FIG. 92 depicts an RF bias voltage waveform in accordance with abias voltage control feature.

[0100]FIG. 93 is a block diagram illustrating a control system forcontrolling bias voltage in accordance with the feature illustrated inFIG. 92.

[0101]FIG. 94 is a top view of a vacuum control valve employed in thereactor of FIG. 85.

[0102]FIG. 95 is a cross-sectional side view of the valve of FIG. 94 inthe closed position.

[0103]FIG. 96 is a side view of the interior surface of the housing ofthe valve of FIG. 95 with an orientation at right angles to that of FIG.95.

[0104]FIG. 97 is a cross-sectional side view of a high voltage wafersupport pedestal useful in the reactor of FIG. 85.

[0105]FIG. 98 is an enlarged cross-sectional view of the wafer supportpedestal of FIG. 97 illustrating a fastener therein.

[0106]FIG. 99 is a block diagram illustrating an ion implantationprocessing system including a plasma immersion ion implantation reactor.

[0107]FIG. 100 is a graph illustrating electron density as a function ofapplied plasma source power for the inductively coupled plasma immersionion implantation reactor of FIG. 79 and the torroidal source plasmaimmersion ion implantation reactor of FIG. 85.

[0108]FIG. 101 is a graph illustrating free fluorine density as afunction of applied plasma source power for the inductively coupledplasma immersion ion implantation reactor of FIG. 79 and the torroidalsource plasma immersion ion implantation reactor of FIG. 85.

[0109]FIG. 102 is a graph illustrating electron density as a function ofapplied plasma source power for the capacitively coupled plasmaimmersion ion implantation reactor of FIG. 83A and the torroidal sourceplasma immersion ion implantation reactor of FIG. 85.

[0110]FIG. 103 is a graph illustrating dopant concentration as afunction of junction depth for different ion energies in the reactor ofFIG. 85 and in a convention ion beam implant machine.

[0111]FIG. 104 is a graph illustrating dopant concentration before andafter post-implant rapid thermal annealing.

[0112]FIG. 105 is a graph illustrating dopant concentration before andafter dynamic surface annealing in the torroidal source plasma immersionion implantation reactor of FIG. 85 and in a convention ion beam implantmachine.

[0113]FIG. 106 is a graph depicting wafer resistivity after ionimplantation and annealing as a function of junction depth obtained withthe reactor of FIG. 85 using dynamic surface annealing and with aconventional ion beam implant machine using rapid thermal annealing.

[0114]FIG. 107 is a graph depicting implanted dopant concentrationobtained with the reactor of FIG. 85 before and after dynamic surfaceannealing.

[0115]FIG. 108 is a graph of RF bias voltage in the reactor of FIG. 85(left ordinate) and of beamline voltage in a beamline implant machine(right ordinate) as a function of junction depth.

[0116]FIG. 109 is a cross-sectional view of the surface of a waferduring ion implantation of source and drain contacts and of thepolysilicon gate of a transistor.

[0117]FIG. 110 is a cross-sectional view of the surface of a waferduring ion implantation of the source and drain extensions of atransistor.

[0118]FIG. 111 is a flow diagram illustrating an ion implantationprocess carried out using the reactor of FIG. 85.

[0119]FIG. 112 is a flow diagram illustrating a sequence of possiblepre-implant, ion implant and possible post implant processes carriedusing the reactor of FIG. 85 in the system of FIG. 99.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Description of aTorroidal Source Reactor

[0120] Referring to FIG. 1, a plasma reactor chamber 100 enclosed by acylindrical side wall 105 and a ceiling 110 houses a wafer pedestal 115for supporting a semiconductor wafer or workpiece 120. A process gassupply 125 furnishes process gas into the chamber 100 through gas inletnozzles 130 a-130 d extending through the side wall 105. A vacuum pump135 controls the pressure within the chamber 100, typically holding thepressure below 0.5 milliTorr (mT). A half-torroidal hollow tubeenclosure or conduit 150 extends above the ceiling 110 in a half circle.The conduit 150, although extending externally outwardly from ceiling110, is nevertheless part of the reactor and forms a wall of thechamber. Internally it shares the same evacuated atmosphere as existselsewhere in the reactor. In fact, the vacuum pump 135, instead of beingcoupled to the bottom of the main part of the chamber as illustrated inFIG. 1, may instead be coupled to the conduit 150. The conduit 150 hasone open end 150 a sealed around a first opening 155 in the reactorceiling 110 and its other end 150 b sealed around a second opening 160in the reactor ceiling 110. The two openings or ports 150, 160 arelocated on generally opposite sides of the wafer support pedestal 115.The hollow conduit 150 is reentrant in that it provides a flow pathwhich exits the main portion of the chamber at one opening and re-entersat the other opening. In this specification, the conduit 150 may bedescribed as being half-torroidal, in that the conduit is hollow andprovides a portion of a closed path in which plasma may flow, the entirepath being completed by flowing across the entire process regionoverlying the wafer support pedestal 115. Notwithstanding the use of theterm torroidal, the trajectory of the path as well as thecross-sectional shape of the path or conduit 150 may be circular ornon-circular, and may be square, rectangular or any other shape either aregular shape or irregular.

[0121] The external conduit 150 may be formed of a relatively thinconductor such as sheet metal, but sufficiently strong to withstand thevacuum within the chamber. In order to suppress eddy currents in thesheet metal of the hollow conduit 150 (and thereby facilitate couplingof an RF inductive field into the interior of the conduit 150), aninsulating gap 152 extends across and through the hollow conduit 150 soas to separate it into two tubular sections. The gap 152 is filled by aring 154 of insulating material such as a ceramic in lieu of the sheetmetal skin, so that the gap is vacuum tight. A second insulating gap 153may be provided, so that one section of the conduit 150 is electricallyfloating. A bias RF generator 162 applies RF bias power to the waferpedestal 115 and wafer 120 through an impedance match element 164.

[0122] The hollow conduit 150 may be formed of a machined metal, such asaluminum or aluminum alloy. Passages for liquid cooling or heating maybe incorporated in the walls of the hollow conduit.

[0123] Alternatively, the hollow conduit 150 may be formed of anon-conductive material instead of the conductive sheet metal. Thenon-conductive material may be a ceramic, for example. In such analternative case, neither gap 152 or 153 is required.

[0124] An antenna 170 such as a winding or coil 165 disposed on one sideof the hollow conduit 150 and wound around an axis parallel to the axisof symmetry of the half-torroidal tube is connected through an impedancematch element 175 to an RF power source 180. The antenna 170 may furtherinclude a second winding 185 disposed on the opposite side of the hollowconduit 150 and wound in the same direction as the first winding 165 sothat the magnetic fields from both windings add constructively.

[0125] Process gases from the chamber 100 fill the hollow conduit 150.In addition, a separate process gas supply 190 may supply process gasesdirectly in to the hollow conduit 150 through a gas inlet 195. The RFfield in the external hollow conduit 150 ionizes the gases in the tubeto produce a plasma. The RF field induced by the circular coil antenna170 is such that the plasma formed in the tube 150 reaches through theregion between the wafer 120 and the ceiling 110 to complete a torroidalpath that includes the half-torroidal hollow conduit 150. As employedherein, the term torroidal refers to the closed and solid nature of thepath, but does not refer or limit its cross-sectional shape ortrajectory, either of which may be circular or non-circular or square orotherwise. Plasma circulates (oscillates) through the complete torroidalpath or region which may be thought of as a closed plasma circuit. Thetorroidal region extends across the diameter of the wafer 120 and, incertain cases, has a sufficient width in the plane of the wafer so thatit overlies the entire wafer surface.

[0126] The RF inductive field from the coil antenna 170 includes amagnetic field which itself is closed (as are all magnetic fields), andtherefore induces a plasma current along the closed torroidal pathdescribed here. It is believed that power from the RF inductive field isabsorbed at generally every location along the closed path, so thatplasma ions are generated all along the path. The RF power absorptionand rate of plasma ion generation may vary among different locationsalong the closed path depending upon a number of factors. However, thecurrent is generally uniform along the closed path length, although thecurrent density may vary. This current alternates at the frequency ofthe RF signal applied to the antenna 170. However, since the currentinduced by the RF magnetic field is closed, the current must beconserved around the circuit of the closed path, so that the amount ofcurrent flowing in any portion of the closed path is generally the sameas in any other portion of the path. As will be described below, thisfact is exploited in the invention to great advantage.

[0127] The closed torroidal path through which the plasma current flowsis bounded by plasma sheaths formed at the various conductive surfacesbounding the path. These conductive surfaces include the sheet metal ofthe hollow conduit 150, the wafer (and/or the wafer support pedestal)and the ceiling overlying the wafer. The plasma sheaths formed on theseconductive surfaces are charge-depleted regions produced as the resultof the charge imbalance due to the greater mobility of the low-massnegative electrons and the lesser mobility of the heavy-mass positiveions. Such a plasma sheath has an electric field perpendicular to thelocal surface underlying the sheath. Thus, the RF plasma current thatpasses through the process region overlying the wafer is constricted byand passes between the two electric fields perpendicular to the surfaceof the ceiling facing the wafer and the surface of the wafer facing thegas distribution plate. The thickness of the sheath (with RF biasapplied to the workpiece or other electrode) is greater where theelectric field is concentrated over a small area, such as the wafer, andis less in other locations such as the sheath covering the ceiling andthe large adjoining chamber wall surfaces. Thus, the plasma sheathoverlying the wafer is much thicker. The electric fields of the waferand ceiling/gas distribution plate sheaths are generally parallel toeach other and perpendicular to the direction of the RF plasma currentflow in the process region.

[0128] When RF power is first applied to the coil antenna 170, adischarge occurs across the gap 152 to ignite a capacitively coupledplasma from gases within the hollow conduit 150. Above a threshold powerlevel, the discharge and plasma current become spatially continuousthrough the length of the hollow conduit 150 and along the entiretorroidal path. Thereafter, as the plasma current through the hollowconduit 150 increases, the inductive coupling of the RF field becomesmore dominant so that the plasma becomes an inductively coupled plasma.Alternatively, plasma may be initiated by other means, such as by RFbias applied to the workpiece support or other electrode or by a sparkor ultraviolet light source.

[0129] In order to avoid edge effects at the wafer periphery, the ports150, 160 are separated by a distance that exceeds the diameter of thewafer. For example, for a 12 inch diameter wafer, the ports 150, 160 areabout 14 to 22 inches apart. For an 8 inch diameter wafer, the ports150, 160 are about 9 to 16 inches apart.

[0130] Notwithstanding the use of the term “wafer”, the workpiece may beany shape, such as rectangular. The workpiece material may be asemiconductor, insulator, or conductor, or a combination of variousmaterials. The workpiece may have 2-dimensional or 3-dimensionalstructure, as well.

Advantages

[0131] A significant advantage is that power from the RF inductive fieldis absorbed throughout the relatively long closed torroidal path (i.e.,long relative to the gap length between the wafer and the reactorceiling), so that RF power absorption is distributed over a large area.As a result, the RF power density in the vicinity of thewafer-to-ceiling gap (i.e., the process region 121 best shown in FIG. 2,not to be confused with the insulating gap 152) is relatively low, thusreducing the likelihood of device damage from RF fields. In contrast, inprior inductively coupled reactors, all of the RF power is absorbedwithin the narrow wafer-to-ceiling gap, so that it is greatlyconcentrated in that region. Moreover, this fact often limits theability to narrow the wafer-to-ceiling gap (in the quest of otheradvantages) or, alternatively, requires greater concentration of RFpower in the region of the wafer. Thus, the invention overcomes alimitation of long standing in the art. This aspect enhances processperformance for some applications by reducing residency time of thereactive gases through a dramatic reduction in volume of the processregion or process zone overlying the wafer, as discussed previouslyherein.

[0132] A related and even more important advantage is that the plasmadensity at the wafer surface can be dramatically increased withoutincreasing the RF power applied to the coil antenna 170 (leading togreater efficiency). This is accomplished by reducing thecross-sectional area of the torroidal path in the vicinity of thepedestal surface and wafer 120 relative to the remainder of thetorroidal path. By so constricting the torroidal path of the plasmacurrent near the wafer only, the density of the plasma near the wafersurface is increased proportionately. This is because the torroidal pathplasma current through the hollow conduit 150 must be at least nearlythe same as the plasma current through the pedestal-to-ceiling(wafer-to-ceiling) gap.

[0133] A significant difference over the prior art is that not only isthe RF field remote from the workpiece, and not only can ion density beincreased at the wafer surface without increasing the applied RF field,but the plasma ion density and/or the applied RF field may be increasedwithout increasing the minimum wafer-to-ceiling gap length. Formerly,such an increase in plasma density necessitated an increase in thewafer-to-ceiling gap to avoid strong fields at the wafer surface. Incontrast, in the present invention the enhanced plasma density isrealized without requiring any increase in the wafer-to-ceiling gap toavoid a concomitant increase in RF magnetic fields at the wafer surface.This is because the RF field is applied remotely from the wafer andmoreover need not be increased to realize an increase in plasma densityat the wafer surface. As a result, the wafer-to-ceiling gap can bereduced down to a fundamental limit to achieve numerous advantages. Forexample, if the ceiling surface above the wafer is conductive, thenreducing the wafer-to-ceiling gap improves the electrical or groundreference provided by the conductive ceiling surface. A fundamentallimit on the minimum wafer-to-ceiling gap length is the sum of thethicknesses of the plasma sheaths on the wafer surface and on theceiling surface.

[0134] A further advantage of the invention is that because the RFinductive field is applied along the entire torroidal path of the RFplasma current (so that its absorption is distributed as discussedabove), the chamber ceiling 110, unlike with most other inductivelypowered reactors, need not function as a window to an inductive fieldand therefore may be formed of any desired material, such as a highlyconductive and thick metal, and therefore may comprise a conductive gasdistribution plate as will be described below, for example. As a result,the ceiling 110 readily provides a reliable electric potential or groundreference across the entire plane of the pedestal or wafer 120.

Increasing the Plasma Ion Density

[0135] One way of realizing higher plasma density near the wafer surfaceby reducing plasma path cross-sectional area over the wafer is to reducethe wafer-to-ceiling gap length. This may be accomplished by simplyreducing the ceiling height or by introducing a conductive gasdistribution plate or showerhead over the wafer, as illustrated in FIG.2. The gas distribution showerhead 210 of FIG. 2 consists of a gasdistribution plenum 220 connected to the gas supply 125 andcommunicating with the process region over the wafer 120 through pluralgas nozzle openings 230. The advantage of the conductive showerhead 210is two-fold: First, by virtue of its close location to the wafer, itconstricts the plasma path over the wafer surface and thereby increasesthe density of the plasma current in that vicinity. Second, it providesa uniform electrical potential reference or ground plane close to andacross the entire wafer surface.

[0136] In order to avoid arcing across the openings 230, each opening230 may be relatively small, on the order of a millimeter (e.g., holediameter is approximately 0.5 mm). The spacing between adjacent openingsmay be on the order of a several millimeters.

[0137] The conductive showerhead 210 constricts the plasma current pathrather than providing a short circuit through itself because a plasmasheath is formed around the portion of the showerhead surface immersedin the plasma. The sheath has a greater impedance to the plasma currentthan the space between the wafer 120 and the showerhead 210, andtherefore virtually all the plasma current goes around the conductiveshowerhead 210.

[0138] It is not necessary to employ a showerhead (e.g., the showerhead210) in order to constrict the torroidal plasma current or path in thevicinity of the process region overlying the wafer. The pathconstriction and consequent increase in plasma ion density in theprocess region may be achieved without the showerhead 210 by similarlyreducing the wafer-to-ceiling height. If the showerhead 210 iseliminated in this manner, then the process gases may be supplied intothe chamber interior by means of conventional gas inlet nozzles, gasdiffusers, or gas slots (not shown).

[0139] One advantage of the showerhead 210 is that different mixtures ofreactive and inert process gas ratios may be introduced throughdifferent orifices 230 at different radii, in order to finely adjust theuniformity of plasma effects on photoresist, for example. Thus, forexample, a greater proportion of inert gas to reactive gas may besupplied to the orifices 230 lying outside a median radius while agreater proportion of reactive gas to inert gas may be supplied to theorifices 230 within that median radius.

[0140] As will be described below, another way in which the torroidalplasma current path may be constricted in the process region overlyingthe wafer (in order to increase plasma ion density over the wafer) is toincrease the plasma sheath thickness on the wafer by increasing the RFbias power applied to the wafer support pedestal. Since as describedpreviously the plasma current across the process region is confinedbetween the plasma sheath at the wafer surface and the plasma sheath atthe ceiling (or showerhead) surface, increasing the plasma sheaththickness at the wafer surface necessarily decreases the cross-sectionof the portion of the torroidal plasma current within process region,thereby increasing the plasma ion density in the process region. Thus,as will be described more fully later in this specification, as RF biaspower on the wafer support pedestal is increased, plasma ion densitynear the wafer surface is increased accordingly.

High Etch Selectivity at High Etch Rates

[0141] The invention solves the problem of poor etch selectivity whichsometimes occurs with a high density plasma. The reactor of FIGS. 1 and2 has a silicon dioxide-to-photoresist etch selectivity as high as thatof a capacitively coupled plasma reactor (about 7:1) while providinghigh etch rates approaching that of a high density inductively coupledplasma reactor. It is believed that the reason for this success is thatthe reactor structure of FIGS. 1 and 2 reduces the degree ofdissociation of the reactive process gas, typically a fluorocarbon gas,so as to reduce the incidence of free fluorine in the plasma region overthe wafer 120. Thus, the proportion of free fluorine in the plasmarelative to other species dissociated from the fluorocarbon gas isdesirably reduced. Such other species include the protective carbon-richpolymer precursor species formed in the plasma from the fluorocarbonprocess gas and deposited on the photoresist as a protective polymercoating. They further include less reactive etchant species such as CFand CF₂ formed in the plasma from the fluorocarbon process gas. Freefluorine tends to attack photoresist and the protective polymer coatingformed thereover as vigorously as it attacks silicon dioxide, thusreducing oxide-to-photoresist etch selectivity. On the other hand, theless reactive etch species such as CF₂ or CF tend to attack photoresistand the protective polymer coating formed thereover more slowly andtherefore provide superior etch selectivity.

[0142] It is believed that the reduction in the dissociation of theplasma species to free fluorine is accomplished in the invention byreducing the residency time of the reactive gas in the plasma. This isbecause the more complex species initially dissociated in the plasmafrom the fluorocarbon process gas, such as CF₂ and CF are themselvesultimately dissociated into simpler species including free fluorine, theextent of this final step of dissociation depending upon the residencytime of the gas in the plasma. The term “residency time” or “residencetime” as employed in this specification corresponds generally to theaverage time that a process gas molecule and the species dissociatedfrom the that molecule are present in the process region overlying theworkpiece or wafer. This time or duration extends from the initialinjection of the molecule into the process region until the moleculeand/or its dissociated progeny are pass out of the process region alongthe closed torroidal path described above that extends through theprocessing zone.

[0143] It is also believed that the reduction in the dissociation of theplasma species to free fluorine is accomplished by reducing the powerdensity of the applied plasma source power as compared to conventionalinductively coupled plasma sources. As stated above, power from the RFinductive field is absorbed throughout the relatively long closedtorroidal path (i.e., long relative to the gap length between the waferand the reactor ceiling), so that RF power absorption is distributedover a large area. As a result, the RF power density in the vicinity ofthe wafer-to-ceiling gap (i.e., the process region 121 best shown inFIG. 2, not to be confused with the insulating gap 152) is relativelylow, thus reducing the dissociation of molecular gases.

[0144] As stated above, the invention enhances etch selectivity byreducing the residency time in the process region of the fluorocarbonprocess gas. The reduction in residency time is achieved by constrictingthe plasma volume between the wafer 120 and the ceiling 110.

[0145] The reduction in the wafer-to-ceiling gap or volume has certainbeneficial effects. First, it increases plasma density over the wafer,enhancing etch rate. Second, residency time falls as the volume isdecreased. As referred to above, the small volume is made possible inthe present invention because, unlike conventional inductively coupledreactors, the RF source power is not deposited within the confines ofthe process region overlying the wafer but rather power deposition isdistributed along the entire closed torroidal path of the plasmacurrent. Therefore, the wafer-to-ceiling gap can be less than a skindepth of the RF inductive field, and in fact can be so small as tosignificantly reduce the residency time of the reactive gases introducedinto the process region, a significant advantage.

[0146] There are two ways of reducing the plasma path cross-section andtherefore the volume over the wafer 120. One is to reduce thewafer-to-showerhead gap distance. The other is to increase the plasmasheath thickness over the wafer by increasing the bias RF power appliedto the wafer pedestal 115 by the RF bias power generator 162, as brieflymentioned above. Either method results in a reduction in free fluorinecontent of the plasma in the vicinity of the wafer 120 (and consequentincrease in dielectric-to-photoresist etch selectivity) as observedusing optical emission spectroscopy (OES) techniques.

[0147] There are three additional methods of the invention for reducingfree fluorine content to improve etch selectivity. One method is tointroduce a non-chemically reactive diluent gas such as argon into theplasma. The argon gas may be introduced outside and above the processregion by injecting it directly into the hollow conduit 150 from thesecond process gas supply 190, while the chemically reactive processgases (fluorocarbon gases) enter the chamber only through the showerhead210. With this advantageous arrangement, the argon ions, neutrals, andexcited neutrals propagate within the torroidal path plasma current andthrough the process region across the wafer surface to dilute the newlyintroduced reactive (e.g., fluorocarbon) gases and thereby effectivelyreduce their residency time over the wafer. Another method of reducingplasma free fluorine content is to reduce the chamber pressure. Afurther method is to reduce the RF source power applied to the coilantenna 170.

[0148]FIG. 3 is a graph illustrating a trend observed in the inventionin which the free fluorine content of the plasma decreases as thewafer-to-showerhead gap distance is decreased. FIG. 4 is a graphillustrating that the free fluorine content of the plasma is decreasedby decreasing the plasma bias power applied to the wafer pedestal 115.FIG. 5 is a graph illustrating that plasma free fluorine content isreduced by reducing the RF source power applied to the coil antenna 170.FIG. 6 is a graph illustrating that the free fluorine content is reducedby reducing chamber pressure. FIG. 7 is a graph illustrating that plasmafree fluorine content is reduced by increasing the diluent (Argon gas)flow rate into the tubular enclosure 150. The graphs of FIGS. 3-7 aremerely illustrative of plasma behavioral trends inferred from numerousOES observations and do not depict actual data.

Wide Process Window

[0149] The chamber pressure is generally less than 0.5 T and can be aslow as 1 mT. The process gas may be C₄F₈ injected into the chamber 100through the gas distribution showerhead at a flow rate of about 15 cc/mwith 150 cc/m of Argon, with the chamber pressure being maintained atabout 20 mT. Alternatively, the Argon gas flow rate may be increased to650 cc/m and the chamber pressure to 60 mT. The antenna 170 may beexcited with about 500 Watts of RF power at 13 MHz. Thewafer-to-showerhead gap may be about 0.3 inches to 2 inches. The bias RFpower applied to the wafer pedestal may be 13 MHz at 2000 Watts. Otherselections of frequency may be made. The source power applied to thecoil antenna 170 may be as low as 50 kHz or as high as several times 13MHz or higher. The same is true of the bias power applied to the waferpedestal.

[0150] The process window for the reactor of FIGS. 1 and 2 is far widerthan the process window for a conventional inductively coupled reactor.This is illustrated in the graph of FIG. 8, showing the specific neutralflux of free fluorine as a function of RF source power for aconventional inductive reactor and for the reactor of FIGS. 1 and 2. Forthe conventional inductively coupled reactor, FIG. 8 shows that the freefluorine specific flux begins to rapidly increase as the source powerexceeds between 50 and 100 Watts. In contrast, the reactor of FIGS. 1and 2 can accept source power levels approaching 1000 Watts before thefree fluorine specific flux begins to increase rapidly. Therefore, thesource power process window in the invention is nearly an order ofmagnitude wider than that of a conventional inductively coupled reactor,a significant advantage.

Dual Advantages

[0151] The constriction of the torroidal plasma current path in thevicinity of the wafer or workpiece produces two independent advantageswithout any significant tradeoffs of other performance criteria: (1) theplasma density over the wafer is increased without requiring anyincrease in plasma source power, and (2) the etch selectivity tophotoresist or other materials is increased, as explained above. It isbelieved that in prior plasma reactors it has been impractical if notimpossible to increase the plasma ion density by the same step thatincreases etch selectivity. Thus, the dual advantages realized with thetorroidal plasma source of the present invention appear to be arevolutionary departure from the prior art.

Other Embodiments

[0152]FIG. 9 illustrates a modification of the case of FIG. 1 in whichthe side antenna 170 is replaced by a smaller antenna 910 that fitsinside the empty space between the ceiling 110 and the hollow conduit150. The antenna 910 is a single coil winding centered with respect tothe hollow conduit 150.

[0153]FIGS. 10 and 11 illustrate how the case of FIG. 1 may be enhancedby the addition of a closed magnetically permeable core 1015 thatextends through the space between the ceiling 110 and the hollow conduit150. The core 1015 improves the inductive coupling from the antenna 170to the plasma inside the hollow conduit 150.

[0154] Impedance match may be achieved without the impedance matchcircuit 175 by using, instead, a secondary winding 1120 around the core1015 connected across a tuning capacitor 1130. The capacitance of thetuning capacitor 1130 is selected to resonate the secondary winding 1120at the frequency of the RF power source 180. For a fixed tuningcapacitor 1130, dynamic impedance matching may be provided by frequencytuning and/or by forward power servoing.

[0155]FIG. 12 illustrates a case of the invention in which a hollow tubeenclosure 1250 extends around the bottom of the reactor and communicateswith the interior of the chamber through a pair of openings 1260, 1265in the bottom floor of the chamber. A coil antenna 1270 follows alongside the torroidal path provided by the hollow tube enclosure 1250 inthe manner of the case of FIG. 1. While FIG. 12 shows the vacuum pump135 coupled to the bottom of the main chamber, it may just as well becoupled instead to the underlying conduit 1250.

[0156]FIG. 13 illustrates a variation of the case of FIGS. 10 and 11, inwhich the antenna 170 is replaced by an inductive winding 1320surrounding an upper section of the core 1015. Conveniently, the winding1320 surrounds a section of the core 1015 that is above the conduit 150(rather than below it). However, the winding 1320 can surround anysection of the core 1015.

[0157]FIG. 14 illustrates an extension of the concept of FIG. 13 inwhich a second hollow tube enclosure 1450 runs parallel to the firsthollow conduit 150 and provides a parallel torroidal path for a secondtorroidal plasma current. The tube enclosure 1450 communicates with thechamber interior at each of its ends through respective openings in theceiling 110. A magnetic core 1470 extends under both tube enclosures150, 1450 and through the coil antenna 170.

[0158]FIG. 15 illustrates an extension of the concept of FIG. 14 inwhich an array of parallel hollow tube enclosures 1250 a, 1250 b, 1250c, 1250 d provide plural torroidal plasma current paths through thereactor chamber. In the case of FIG. 15, the plasma ion density iscontrolled independently in each individual hollow conduit 1250 a-d byan individual coil antenna 170 a-d, respectively, driven by anindependent RF power source 180 a-d, respectively. Individualcylindrical open cores 1520 a-1520 d may be separately inserted withinthe respective coil antennas 170 a-d. In this case, the relativecenter-to-edge ion density distribution may be adjusted by separatelyadjusting the power levels of the individual RF power sources 180 a-d.

[0159]FIG. 16 illustrates a modification of the case of FIG. 15 in whichthe array of tube enclosures 1250 a-d extend through the side wall ofthe reactor rather than through the ceiling 110. Another modificationillustrated in FIG. 16 is the use of a single common magnetic core 1470adjacent all of the tube enclosures 1250 a-d and having the antenna 170wrapped around it so that a single RF source excites the plasma in allof the tube enclosures 1250 a-d.

[0160]FIG. 17A illustrates a pair of orthogonal tube enclosures 150-1and 150-2 extending through respective ports in the ceiling 110 andexcited by respective coil antennas 170-1 and 170-2. Individual cores1015-1 and 1015-2 are within the respective coil antennas 170-1 and170-2. This case creates two mutually orthogonal torroidal plasmacurrent paths over the wafer 120 for enhanced uniformity. The twoorthogonal torroidal or closed paths are separate and independentlypowered as illustrated, but intersect in the process region overlyingthe wafer, and otherwise do not interact. In order to assure separatecontrol of the plasma source power applied to each one of the orthogonalpaths, the frequency of the respective RF generators 180 a, 180 b ofFIG. 17 are different, so that the operation of the impedance matchcircuits 175 a, 175 b is decoupled. For example, the RF generator 180 amay produce an RF signal at 11 MHz while the RF generator 180 b mayproduce an RF signal at 12 MHz. Alternatively, independent operation maybe achieved by offsetting the phases of the two RF generators 180 a, 180b.

[0161]FIG. 17B illustrates how radial vanes 181 may be employed to guidethe torroidal plasma currents of each of the two conduits 150-1, 150-2through the processing region overlying the wafer support. The radialvanes 181 extend between the openings of each conduit near the sides ofthe chamber up to the edge of the wafer support. The radial vanes 181prevent diversion of plasma from one torroidal path to the othertorroidal path, so that the two plasma currents only intersect withinthe processing region overlying the wafer support.

Cases Suitable for Large Diameter Wafers

[0162] In addition to the recent industry trends toward smaller devicesizes and higher device densities, another trend is toward greater waferdiameters. For example, 12-inch diameter wafers are currently enteringproduction, and perhaps larger diameter wafers will be in the future.The advantage is greater throughput because of the large number ofintegrated circuit die per wafer. The disadvantage is that in plasmaprocessing it is more difficult to maintain a uniform plasma across alarge diameter wafer. The following cases of the present invention areparticularly adapted for providing a uniform plasma ion densitydistribution across the entire surface of a large diameter wafer, suchas a 12-inch diameter wafer.

[0163]FIGS. 18 and 19 illustrate a hollow tube enclosure 1810 which is awide flattened rectangular version 1850 of the hollow conduit 150 ofFIG. 1 that includes an insulating gap 1852. This version produces awide “belt” of plasma that is better suited for uniformly covering alarge diameter wafer such as a 12-inch diameter wafer or workpiece. Thewidth W of the tube enclosure and of the pair of openings 1860, 1862 inthe ceiling 110 may exceed the wafer by about 5% or more. For example,if the wafer diameter is 10 inches, then the width W of the rectangulartube enclosure 1850 and of the openings 1860, 1862 is about 11 inches.FIG. 20 illustrates a modified version 1850′ of the rectangular tubeenclosure 1850 of FIGS. 18 and 19 in which a portion 1864 of theexterior tube enclosure 1850 is constricted.

[0164]FIG. 20 further illustrates the optional use of focusing magnets1870 at the transitions between the constricted and unconstrictedportions of the enclosure 1850. The focusing magnets 1870 promote abetter movement of the plasma between the constricted and unconstrictedportions of the enclosure 1850, and specifically promote a more uniformspreading out of the plasma as it moves across the transition betweenthe constricted portion 1864 and the unconstricted portion of the tubeenclosure 1850.

[0165]FIG. 21 illustrates how plural cylindrical magnetic cores 2110 maybe inserted through the exterior region 2120 circumscribed by the tubeenclosure 1850. The cylindrical cores 2110 are generally parallel to theaxis of symmetry of the tube enclosure 1850. FIG. 22 illustrates amodification of the case of FIG. 21 in which the cores 2110 extendcompletely through the exterior region 2120 surrounded by the tubeenclosure 1850 are replaced by pairs of shortened cores 2210, 2220 inrespective halves of the exterior region 2120. The side coils 165, 185are replaced by a pair of coil windings 2230, 2240 surrounding therespective core pairs 2210, 2220. In this case, the displacement Dbetween the core pairs 2210, 2220 may be changed to adjust the iondensity near the wafer center relative to the ion density at the wafercircumference. A wider displacement D reduces the inductive couplingnear the wafer center and therefore reduces the plasma ion density atthe wafer center. Thus, an additional control element is provided forprecisely adjusting ion density spatial distribution across the wafersurface. FIG. 23 illustrates a variation of the case of FIG. 22 in whichthe separate windings 2230, 2240 are replaced by a single center winding2310 centered with respect to the core pairs 2210, 2220.

[0166]FIGS. 24 and 25 illustrate a case providing even greateruniformity of plasma ion density distribution across the wafer surface.In the case of FIGS. 24 and 25, two torroidal plasma current paths areestablished that are transverse to one another and are mutuallyorthogonal. This is accomplished by providing a second wide rectangularhollow enclosure 2420 extending transversely and orthogonally relativeto the first tube enclosure 1850. The second tube enclosure 2420communicates with the chamber interior through a pair of openings 2430,2440 through the ceiling 110 and includes an insulating gap 2452. A pairof side coil windings 2450, 2460 along the sides of the second tubeenclosure 2420 maintain a plasma therein and are driven by a second RFpower supply 2470 through an impedance match circuit 2480. As indicatedin FIG. 24, the two orthogonal plasma currents coincide over the wafersurface and provide more uniform coverage of plasma over the wafersurface. This case is expected to find particularly advantageous use forprocessing large wafers of diameters of 10 inches and greater.

[0167] As in the case of FIG. 17, the case of FIG. 24 creates twomutually orthogonal torroidal plasma current paths over the wafer 120for enhanced uniformity. The two orthogonal torroidal or closed pathsare separate and independently powered as illustrated, but intersect inthe process region overlying the wafer, and otherwise do not interact orotherwise divert or diffuse one another. In order to assure separatecontrol of the plasma source power applied to each one of the orthogonalpaths, the frequency of the respective RF generators 180, 2470 of FIG.24 are different, so that the operation of the impedance match circuits175, 2480 is decoupled. For example, the RF generator 180 may produce anRF signal at 11 MHz while the RF generator 2470 may produce an RF signalat 12 MHz. Alternatively, independent operation may be achieved byoffsetting the phases of the two RF generators 180, 2470.

[0168]FIG. 26 illustrates a variation of the case of FIG. 18 in which amodified rectangular enclosure 2650 that includes an insulating gap 2658communicates with the chamber interior through the chamber side wall 105rather than through the ceiling 110. For this purpose, the rectangularenclosure 2650 has a horizontal top section 2652, a pair of downwardlyextending legs 2654 at respective ends of the top section 2652 and apair of horizontal inwardly extending legs 2656 each extending from thebottom end of a respective one of the downwardly extending legs 2654 toa respective opening 2670, 2680 in the side wall 105.

[0169]FIG. 27 illustrates how a second rectangular tube enclosure 2710including an insulating gap 2752 may be added to the case of FIG. 26,the second tube enclosure 2710 being identical to the rectangular tubeenclosure 2650 of FIG. 26 except that the rectangular tube enclosures2650, 2710 are mutually orthogonal (or at least transverse to oneanother). The second rectangular tube enclosure communicates with thechamber interior through respective openings through the side wall 105,including the opening 2720. Like the case of FIG. 25, the tubeenclosures 2650 and 2710 produce mutually orthogonal torroidal plasmacurrents that coincide over the wafer surface to provide superioruniformity over a broader wafer diameter. Plasma source power is appliedto the interior of the tube enclosures through the respective pairs ofside coil windings 165, 185 and 2450, 2460.

[0170]FIG. 28A illustrates how the side coils 165, 185, 2450, 2460 maybe replaced (or supplemented) by a pair of mutually orthogonal interiorcoils 2820, 2840 lying within the external region 2860 surrounded by thetwo rectangular tube enclosures 2650, 2710. Each one of the coils 2820,2840 produces the torroidal plasma current in a corresponding one of therectangular tube enclosures 2650, 2710. The coils 2820, 2840 may bedriven completely independently at different frequencies or at the samefrequency with the same or a different phase. Or, they may be driven atthe same frequency but with a phase difference (i.e., 90 degrees) thatcauses the combined torroidal plasma current to rotate at the sourcepower frequency. In this case the coils 2820, 2840 are driven with thesin and cosine components, respectively, of a common signal generator2880, as indicated in FIG. 28A. The advantage is that the plasma currentpath rotates azimuthally across the wafer surface at a rotationalfrequency that exceeds the plasma ion frequency so that non-uniformitiesare better suppressed than in prior art methods such as MERIE reactorsin which the rotation is at a much lower frequency.

[0171] Referring now to FIG. 28B, radial adjustment of plasma iondensity may be generally provided by provision of a pair of magneticcylindrical cores 2892, 2894 that may be axially moved toward or awayfrom one another within the coil 2820 and a pair of magnetic cylindricalcores 2896, 2898 that may be axially moved toward or away from oneanother within the coil 2840. As each pair of cores is moved toward oneanother, inductive coupling near the center of each of the orthogonalplasma currents is enhanced relative to the edge of the current, so thatplasma density at the wafer center is generally enhanced. Thus, thecenter-to-edge plasma ion density may be controlled by moving the cores2892, 2894, 2896, 2898.

[0172]FIG. 29 illustrates an alternative case of the invention in whichthe two tube enclosures 2650, 2710 are merged together into a singleenclosure 2910 that extends 360 degrees around the center axis of thereactor that constitutes a single plenum. In the case of FIG. 29, theplenum 2910 has a half-dome lower wall 2920 and a half-dome upper wall2930 generally congruent with the lower wall 2920. The plenum 2910 istherefore the space between the upper and lower half-dome walls 2920,2930. An insulating gap 2921 may extend around the upper dome wall 2920and/or an insulating gap 2931 may extend around the lower dome wall2930. The plenum 2910 communicates with the chamber interior through anannular opening 2925 in the ceiling 110 that extends 360 degrees aroundthe axis of symmetry of the chamber.

[0173] The plenum 2910 completely encloses a region 2950 above theceiling 110. In the case of FIG. 29, plasma source power is coupled intothe interior of the plenum 2910 by a pair of mutually orthogonal coils2960, 2965. Access to the coils 2960, 2965 is provided through avertical conduit 2980 passing through the center of the plenum 2910.Preferably, the coils 2960, 2965 are driven in quadrature as in the caseof FIG. 28 to achieve an azimuthally circulating torroidal plasmacurrent (i.e., a plasma current circulating within the plane of thewafer. The rotation frequency is the frequency of the applied RF power.Alternatively, the coils 2960, 2965 may be driven separately atdifferent frequencies. FIG. 30 is a top sectional view of the case ofFIG. 29. FIGS. 31A and 31B are front and side sectional views,respectively, corresponding to FIG. 30.

[0174] The pair of mutually orthogonal coils 2960, 2965 may be replacedby any number n of separately driven coils with their winding axesdisposed at 360/n degrees apart. For example, FIG. 32 illustrates thecase where the two coils 2960, 2965 are replace by three coils 3210,3220, 3230 with winding axes placed at 120 degree intervals and drivenby three respective RF supplies 3240, 3250, 3260 through respectiveimpedance match circuits 3241, 3251, 3261. In order to produce arotating torroidal plasma current, the three windings 3210, 3220, 3230are driven 120 degrees out of phase from a common power source 3310 asillustrated in FIG. 33. The cases of FIGS. 32 and 33 are preferred overthe case of FIG. 29 having only two coils, since it is felt much of themutual coupling between coils would be around rather than through thevertical conduit 2980.

[0175]FIG. 34 illustrates a case in which the three coils are outside ofthe enclosed region 2950, while their inductances are coupled into theenclosed region 2950 by respective vertical magnetic cores 3410extending through the conduit 2980. Each core 3410 has one end extendingabove the conduit 2980 around which a respective one of the coils 3210,3220, 3230 is wound. The bottom of each core 3410 is inside the enclosedregion 2950 and has a horizontal leg. The horizontal legs of the threecores 3410 are oriented at 120 degree intervals to provide inductivecoupling to the interior of the plenum 2910 similar to that provided bythe three coils inside the enclosed region as in FIG. 32.

[0176] The advantage of the flattened rectangular tube enclosures of thecases of FIGS. 18-28 is that the broad width and relatively low heightof the tube enclosure forces the torroidal plasma current to be a widethin belt of plasma that more readily covers the entire surface of alarge diameter wafer. The entirety of the tube enclosure need not be ofthe maximum width. Instead the outer section of the tube enclosurefarthest from the chamber interior may be necked down, as discussedabove with reference to the case of FIG. 20. In this case, it ispreferable to provide focusing magnets 1870 at the transition cornersbetween the wide portion 1851 and the narrow section 1852 to force theplasma current exiting the narrow portion 1852 to spread entirely acrossthe entire width of the wide section 1851. If it is desired to maximizeplasma ion density at the wafer surface, then it is preferred that thecross-sectional area of the narrow portion 1852 be at least nearly asgreat as the cross-sectional area of the wide portion 1851. For example,the narrow portion 1852 may be a passageway whose height and width areabout the same while the wide portion 1851 may have a height that isless than its width.

[0177] The various cases described herein with air-core coils (i.e.,coils without a magnetic core) may instead employ magnetic-cores, whichcan be the open-magnetic-path type or the closed-magnetic-core typeillustrated in the accompanying drawings. Furthermore, the various casesdescribed herein having two or more torroidal paths driven withdifferent RF frequencies may instead be driven with same frequency, andwith the same or different phases.

[0178]FIG. 35 is a version of the case of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the case of FIG. 20.

[0179]FIG. 36 is a version of the case of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

[0180]FIG. 37 is a case corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber. Having a number of symmetrically disposed conduitsand reentrant ports greater than two (as in the case of FIG. 37) isbelieved to be particularly advantageous for processing wafers ofdiameter of 300 mm and greater.

[0181]FIG. 38 is a case corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

[0182]FIG. 39 is a case corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

[0183]FIG. 40 is a case corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

[0184]FIG. 41 is a case corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

[0185]FIG. 42 is a case corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

[0186]FIG. 43 is a case corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

Advantageous Features

[0187] Constricting the torroidal plasma current in the vicinity of thewafer not only improves etch selectivity but at the same time increasesthe etch rate by increasing the plasma ion density. It is believed noprior reactor has increased etch selectivity by the same mechanism thatincreases etch rate or plasma ion density over the workpiece.

[0188] Improving etch selectivity by constricting the torroidal plasmacurrent in the vicinity of the wafer or workpiece can be achieved in theinvention in any one of several ways. One way is to reduce thepedestal-to-ceiling or wafer-to-ceiling height. Another is to introducea gas distribution plate or showerhead over the wafer that constrictsthe path of the torroidal plasma ion current. Another way is to increasethe RF bias power applied to the wafer or workpiece. Any one or anycombination of the foregoing ways of improving etch selectivity may bechosen by the skilled worker in carrying out the invention.

[0189] Etch selectivity may be further improved in the invention byinjecting the reactive process gases locally (i.e., near the wafer orworkpiece) while injecting an inert diluent gas (e.g., Argon) remotely(i.e., into the conduit or plenum). This may be accomplished byproviding a gas distribution plate or showerhead directly over andfacing the workpiece support and introducing the reactive process gasexclusively (or at least predominantly) through the showerhead.Concurrently, the diluent gas is injected into the conduit well awayfrom the process region overlying the wafer or workpiece. The torroidalplasma current thus becomes not only a source of plasma ions forreactive ion etching of materials on the wafer but, in addition, becomesan agent for sweeping away the reactive process gas species and theirplasma-dissociated progeny before the plasma-induced dissociationprocess is carried out to the point of creating an undesirable amount offree fluorine. This reduction in the residence time of the reactiveprocess gas species enhances the etch selectivity relative tophotoresist and other materials, a significant advantage.

[0190] Great flexibility is provided in the application of RF plasmasource power to the torroidal plasma current. As discussed above, poweris typically inductively coupled to the torroidal plasma current by anantenna. In many cases, the antenna predominantly is coupled to theexternal conduit or plenum by being close or next to it. For example, acoil antenna may extend alongside the conduit or plenum. However, inother cases the antenna is confined to the region enclosed between theconduit or plenum and the main reactor enclosure (e.g., the ceiling). Inthe latter case, the antenna may be considered to be “under” the conduitrather than alongside of it. Even greater flexibility is afford by caseshaving a magnetic core (or cores) extending through the enclosed region(between the conduit and the main chamber enclosure) and having anextension beyond the enclosed region, the antenna being wound around thecore's extension. In this case the antenna is inductively coupled viathe magnetic core and therefore need not be adjacent the torroidalplasma current in the conduit. In one such case, a closed magnetic coreis employed and the antenna is wrapped around the section of the corethat is furthest away from the torroidal plasma current or the conduit.Therefore, in effect, the antenna may be located almost anywhere, suchas a location entirely remote from the plasma chamber, by remotelycoupling it to the torroidal plasma current via a magnetic core.

[0191] Finally, plasma distribution over the surface of a very largediameter wafer or workpiece is uniform. This is accomplished in one caseby shaping the torroidal plasma current as a broad plasma belt having awidth preferably exceeding that of the wafer. In another case,uniformity of plasma ion density across the wafer surface is achieved byproviding two or more mutually transverse or orthogonal torroidal plasmacurrents that intersect in the process region over the wafer. Thetorroidal plasma currents flow in directions mutually offset from oneanother by 360/n. Each of the torroidal plasma currents may be shaped asa broad belt of plasma to cover a very large diameter wafer. Each one ofthe torroidal plasma currents may be powered by a separate coil antennaaligned along the direction of the one torroidal plasma current. In onepreferred case, uniformity is enhanced by applying RF signals ofdifferent phases to the respective coil antennas so as to achieve arotating torroidal plasma current in the process region overlying thewafer. In this preferred case, the optimum structure is one in which thetorroidal plasma current flows in a circularly continuous plenumcommunicating with the main chamber portion through a circularlycontinuous annular opening in the ceiling or side wall. This latterfeature allows the entire torroidal plasma current to rotate azimuthallyin a continuous manner.

Controlling Radial Distribution of Plasma Ion Density

[0192]FIG. 44 illustrates a plasma reactor similar to that illustratedin FIG. 17A having a pair of orthogonal external reentrant tubes 150-1,150B2. RF power is coupled into the tubes by respective annular magneticcores 1015-1, 1015-2 excited by respective RF-driven coils 170-1, 170-2,as described above with reference to FIG. 17A. However, in FIG. 44 theexternal tubes 150-1, 150-2 are rectangular as in FIG. 24 rather thanbeing round in cross-sectional shape. Moreover, the horizontal sectionof the lower tube 150-1 is not flat but rather has a dip 4410 at itsmiddle. The dip 4410 permits the upper external tube 150-2 to nestcloser to the reactor ceiling 110. This feature shortens the path lengthin the upper tube 150-2, thereby reducing plasma losses in the uppertube 150-2. In fact, the shape of the dip 4410 may be selected to atleast nearly equalize the path length through the upper and lowerexternal tubes 150-1, 150-2. The reactor of FIG. 44, like the reactorsof FIGS. 2 and 26, has a gas distribution plate 210 on the ceiling 110(or forming the ceiling 110 itself) and overlying the wafer 120.

[0193] The dip 4410 is limited in that a vertical space remains betweenthe top surface of the ceiling 110 and a bottom corner 4422 formed onthe lower tube 150-1 at the apex of the dip 4410. The vertical spaceaccommodates an electromagnet assembly 4430 that enhances plasma iondensity over the center of the wafer 120. The electromagnet assembly4430 includes a narrow elongate cylindrical pole piece 4440 formed of amagnetizable metal such as iron or steel (for example) and a coil 4450of insulated conductive wire (e.g., copper wire) wrapped around the polepiece 4440. The cylindrical axis of the pole piece 4440 coincides withthe axis of symmetry of the cylindrical chamber 100, so that the axis ofthe pole piece 4440 intersects the center of the wafer 120. The coil4450 may be wrapped directly on the pole piece 4440 or, as illustratedin FIG. 45, may be wrapped around a mandril 4460 encircling the polepiece 4440. FIG. 45 shows that the coil 4450 may be wrapped around asection 4440-1 of the pole piece 4440 that extends above the ceiling110. The lower section 4440-2 of the pole piece 4440 that is inside theceiling 110 terminates within the gas manifold 220 of the gasdistribution plate 210.

[0194] For efficiency, it is desirable to place the source of theplasma-confining magnetic field as close to the plasma as practicalwithout disturbing gas flow within the gas distribution plate 210. Forthis purpose, the portion of the lower pole piece section 4440-2 that isinside the gas manifold 220 is a very narrow cylindrical end piece 4470that terminates the pole piece 4440. The end piece 4470 extends themagnetic field lines of the pole piece 4440 near the bottom of the gasdistribution plate to enhance the effect of the magnetic field on theplasma. The diameter of the end piece 4470 is sufficiently reduced sothat it does not appreciably interfere with gas flow within the gasmanifold 210. Moreover, such a reduced diameter brings the peak of theradial component of the magnetic field nearer the center axis.

[0195]FIG. 46 illustrates one case of the end piece 4470 having atapered bottom 4475 terminated in a nipple 4477. FIG. 47 illustrates acase of the end piece 4470 in which the bottom 4476 is flat. FIG. 48illustrates a case of the end piece 4470 in which the bottom 4478 isround.

[0196] In one implementation, pole piece 4440 has a diameter of about3.5 cm (such that the diameter of the approximately 60 turn coil 4450 isabout 6 cm) and is about 12 cm long. The pole piece 4440 is extendedabout 2 cm (to a total of about 14 cm) with a smaller diameter extensionof about 1 cm diameter. The bottom of the extension region of the polepiece 4440 is about 1.5 cm from the top of the plasma region. Thematerial composition of pole piece 4440 is selected to have sufficientlyhigh permeability (e.g., μr>or=100) and high saturation flux density(e.g. Bsa>1000 gauss) to maximize the magnetic flux density in theregion below the pole piece 4440 with minimum magnetizing force andcurrent. Note that because the magnetic path is “open” with pole piece4440 (not closed within the pole piece), the effective permeability isreduced relative to the material permeability. Depending on thelength/diameter ratio of the pole piece 4440, the μr “effective” istypically reduced to on the order of 10.

[0197] An optional shield 4479 of magnetic material such as iron shieldsplasma in the pair of tubes 150-1, 150-2 from the D.C. magnetic field ofthe electromagnet assembly 4430. The shield 4479 includes an overheadplate 4479 a and a cylindrical skirt 4479 b.

[0198] In the case of the gas distribution plate 210 illustrated in FIG.45, a top plate 4480 is divided into radially inner and outer sections4480 a, 4480 b, each having many small gas flow holes 4481 extendingthrough it, the inner and outer sections having annular flanges 4482-1,4482-2, 4482-3, 4482-4, forming vertical walls supporting the bottomsurface of the ceiling 210 and forming therewith inner and outer gasmanifolds 4483 a, 4483 b separated by a wall formed by the annularflanges 4482-2, 4482-3. In one case, there is no wall between the innerand outer gas manifolds, so as to avoid any discontinuity in gasdistribution within the chamber that such a wall may cause. A gas mixinglayer 4484 below the top plate 4480 diverts gas flow from a purelyvertical flow direction and thereby induces multi-directional (orturbulent) gas flow that improves uniform mixing of gases of differentmolecular weights. Such diverting of the gas flow from a purely downwardflow direction has the added benefit of suppressing high velocity gasflow effects, in which high velocity gas flow through gas distributionplate orifices directly over the wafer would form localizedconcentrations of process gas on the wafer surface that disrupt processuniformity. Suppression of high velocity gas flow effects enhancesuniformity.

[0199] The gas mixing layer 4484 may consist of metal or ceramic foam ofthe type well-known in the art. Or, as shown in FIG. 49, the gas mixturelayer 4484 may consist of plural perforation plates 4484-1, 4484-2 eachhaving many small gas orifices drilled through it, the holes in oneperforation plate being offset from the holes in the other perforationplate. A bottom plate 4485 of the gas distribution plate 210 has manysub-millimeter gas injection holes 4486 drilled through it with largecounterbored holes 4487 at the top of the bottom plate 4485. In oneexample, the sub-millimeter holes were between 10 and 30 mils indiameter, the counterbored holes were about 0.06 inch in diameter andthe bottom plate 4485 had a thickness of about 0.4 inch. Inner and outergas feed lines 4490, 4492 through the ceiling 110 furnish gas to theinner and outer top plates 4480 a, 4480 b, so that gas flow in radiallyinner and outer zones of the chamber may be controlled independently asa way of adjusting process uniformity.

[0200] It is believed that the radial component of the D.C. magneticfield produced by the electromagnet assembly 4430 affects the radialdistribution of plasma ion density, and that it is this radial componentof the magnetic field that can be exploited to enhance plasma iondensity near the center of the chamber. It is believed that suchenhancement of plasma ion density over the wafer center arises from theinteraction of the D.C. magnetic field radial component with the plasmasheath electric field at the wafer surface producing azimuthal plasmacurrents tending to confine plasma near the wafer center. In absence ofthe D.C. magnetic field, the phenomenon of a reduced plasma ion densityat the center of the chamber extends over a very small circular zoneconfined closely to the center of the wafer 120, because in general thereactor of FIG. 44 tends to have an exceptionally uniform plasma iondensity even in absence of a correcting magnetic field. Therefore,correction of the center-low plasma ion density distribution requires aD.C. magnetic field having a relatively large radial component very nearthe center of the chamber or wafer 120. The small diameter of themagnetic pole piece 4440 produces a magnetic field having a large radialcomponent very close to the center of the wafer 120 (or center of thechamber). In accordance with conventional practice, the center is theaxis of symmetry of the cylindrical chamber at which the radius is zero.FIG. 51 illustrates the distribution of the magnetic field in anelevational view of the processing region over the wafer 120 between thewafer 120 and the gas distribution plate 210. The vectors in FIG. 51 arenormalized vectors representing the direction of the magnetic field atvarious locations. FIG. 52 illustrates the magnetic flux density of theradial component of the magnetic field as a function of radial location,one curve representing the radial field flux density near the bottomsurface of the gas distribution plate 210 and the other curverepresenting the radial field flux density near the surface of the wafer120. The peak of the flux density of the radial magnetic field componentis very close to the center, namely at about a radius of only one inchboth at the ceiling and at the wafer. Thus, the radial component of themagnetic field is tightly concentrated near the very small diameterregion within which the plasma ion density tends to be lowest. Thus, thedistribution of the radial component of the D.C. magnetic field producedby the electromagnet assembly 4430 generally coincides with the regionof low plasma ion density near the center of the chamber.

[0201] As mentioned above, it is felt that the radial component of theD.C. magnetic field interacts with the vertically oriented electricfield of the plasma sheath near the wafer center to produce anazimuthally directed force that generally opposes radial travel ofplasma. As a result, plasma near the center of the wafer is confined toenhance processing within that region.

[0202] A basic approach of using the electromagnet assembly 4430 in anetch reactor is to find a D.C. current flow in the coil that producesthe most uniform etch rate radial distribution across the wafer surface,typically by enhancing plasma ion density at the center. This is thelikeliest approach in cases in which the wafer-to-ceiling gap isrelatively small (e.g., one inch), since such a small gap typicallyresults in a center-low etch rate distribution on the wafer. Forreactors having a larger gap (e.g., two inches or more), the etch ratedistribution may not be center low, so that a different D.C. current maybe needed. Of course, the electromagnet assembly 4430 is not confined toapplications requiring improved uniformity of plasma ion density acrossthe wafer surface. Some applications of the electromagnet assembly mayrequire an electromagnet coil current that renders the plasma iondensity less uniform. Such applications may involve, for example, casesin which a field oxide thin film layer to be etched has a non-uniformthickness distribution, so that uniform results can be obtained only byproviding nonuniform plasma ion density distribution that compensatesfor the nonuniform field oxide thickness distribution. In such a case,the D.C. current in the electromagnet assembly can be selected toprovide the requisite nonuniform plasma ion distribution.

[0203] As shown in FIG. 45, the plasma reactor may include a set ofintegrated rate monitors 4111 that can observe the etch ratedistribution across the wafer 120 during the etch process. Each monitor4111 observes the interference fringes in light reflected from thebottom of contact holes while the holes are being etched. The light canbe from a laser or may be the luminescence of the plasma. Such real timeobservation can make it possible to determine changes in etch ratedistribution across the wafer that can be instantly compensated bychanging the D.C. current applied to the electromagnet assembly 4430.

[0204]FIG. 53 shows one way of independently controlling process gasflow to the inner and outer gas feed lines 4490, 4492. In FIG. 53, oneset of gas flow controllers 5310, 5320, 5330 connected to the inner gasfeed line 4490 furnish, respectively, argon, oxygen and a fluoro-carbongas, such as C4F6, to the inner gas feed line 4490. Another set of gasflow controllers 5340, 5350, 5360 furnish, respectively, argon, oxygenand a fluoro-carbon gas, such as C4F6, to the outer gas feed line 4492.FIG. 54 shows another way of independently controlling process gas flowto the inner and outer gas feed lines 4490, 4492. In FIG. 54, a singleset of gas flow controllers 5410, 5420, 5430 furnishes process gases(e.g., argon, oxygen and a fluoro-carbon gas) to a gas splitter 5440.The gas splitter 5440 has a pair of gas or mass flow controllers (MFC's)5442, 5444 connected, respectively, to the inner and outer gas feedlines 4490, 4492. In addition, optionally another gas flow controller5446 supplies purge gas such as Argon or Neon to the outer gas feed line4492.

[0205] One problem in processing a large diameter wafer is that thetorroidal or reentrant plasma current must spread out evenly over thewide surface of the wafer. The tubes 150 typically are less wide thanthe process area. The need then is to broaden the plasma current tobetter cover a wide process area as it exits a port 155 or 160. Asrelated problem is that the reactor of FIG. 44 (or any of the reactorsof FIGS. 1-43) can experience a problem of non-uniform plasma iondensity and consequent “hot spot” or small region 5505 of very highplasma ion density near a port 155 or 160 of the reentrant tube 150, asshown in FIG. 55A. Referring to FIGS. 55A-56B, these problems areaddressed by the introduction of a plasma current flow splitter 5510 atthe mouth of each port (e.g., the port 155 as shown in FIG. 55A). Thesplitter 5510 tends to force the plasma current to widen while at thesame time reducing plasma ion density in the vicinity of the region 5505where a hot spot might otherwise form. The tube 150 can have a widenedtermination section 5520 at the port 155, the termination section 5520having a diameter nearly twice as great as that of the remaining portionof the tube 150. The plasma current flow splitter 5510 of FIG. 55A istriangular in shape, with one apex facing the interior of the tube 150so as to force the plasma current flowing into the chamber 100 from thetube 150 to spread out so as to better fill the larger diameter of thetermination section 5520. This current-spreading result produced by thetriangular splitter 5510 tends to widen the plasma current and reducesor eliminates the “hot spot” in the region 5505.

[0206] The optimum shape of the splitter 5510 depends at least in partupon the separation distance S between the centers of opposing ports155, 160. If the splitter is too long in the direction of plasma flow(i.e., the vertical direction in FIG. 55A), then current flow along thedivided path tends to be unbalanced, with all current flowing along oneside of the splitter 5510. On the other hand, if the splitter 5510 istoo short, the two paths recombine before the plasma current appreciablywidens.

[0207] For example, in a chamber for processing a 12-inch diameterwafer, the separation distance S can be about 20.5 inches, with a tubewidth w of 5 inches, a tube draft d of 1.75 inches and an expandedtermination section width W of 8 inches. In this case, the juxtapositionof the port 155 relative to the 12 inch wafer would be as shown in theplan view of FIG. 56C. In this particular example, the height h of thesplitter 5510 should be about 2.5 inches, with the angle of thesplitter's apex 5510 a being about 75 degrees, as shown in FIG. 57. Inaddition, the length L of the termination section 5520 should equal theheight h of the splitter 5510.

[0208] On the other hand, for a separation distance S of 16.5 inches, anoptimum splitter 5510′ is illustrated in FIG. 58. The angle of thesplitter apex in this case is preferably about 45 degrees, thetriangular portion being terminated in a rectangular portion having awidth of 1.2 inches and a length such that the splitter 5510′ has aheight h of 2.5 inches. The height and apex angle of the splitter 5510or 5510′ must be sufficient to reduce plasma density in the region 5505to prevent formation of a hot spot there. However, the height h must belimited in order to avoid depleting plasma ion density at the wafercenter.

[0209]FIGS. 59A and 59B illustrate splitters for solving the problem ofplasma ion density non-uniformity near the entrance ports of a reentranttube 2654 in which plasma current flow through each port is in ahorizontal direction through the chamber side wall 105, as in thereactor of FIG. 26. Each splitter 5910 has its apex 5910 a facing theport 2680.

[0210]FIGS. 60, 61 and 62 illustrate an implementation like that of FIG.17A, except that the chamber side wall 105 is rectangular or square andthe vertically facing ports 140-1, 140-2, 140-3 and 140-4 through theceiling 110 are located over respective corners 105 a, 105 b, etc. ofthe rectangular or square side wall 105. A floor 6020 in the plane ofthe wafer 120 faces each port and, together with the corner-formingsections of the rectangular side wall 105, forces incoming plasmacurrent to turn toward the processing region overlying the wafer 120. Inorder to reduce or eliminate a hot spot in plasma ion density in theregion 6030, a triangular plasma current flow splitter 6010 is placednear each respective corner 105 a, 105 b, etc., with its apex 6010 afacing that corner. In the implementation of FIG. 61, the splitter apex6010 a is rounded, but in other implementations it may be less roundedor actually may be a sharp edge. FIG. 63 illustrates a portion of thesame arrangement but in which the edge 6010 b of the splitter 6010facing the wafer 120 is located very close to the wafer 120 and isarcuately shaped to be congruent with the circular edge of the wafer120. While the splitter 6010 of FIG. 60 extends from the floor 6020 tothe ceiling 110, FIG. 64 illustrates that the height of the splitter6010 may be less, so as to allow some plasma current to pass over thesplitter 6010.

[0211] As will be discussed in greater detail below with respect tocertain working examples, the total path length traversed by thereentrant plasma current affects plasma ion density at the wafersurface. This is because shorter path length places a higher proportionof the plasma within the processing region overlying the wafer, reducespath length-dependent losses of plasma ions and reduces surface arealosses due to plasma interaction with the reentrant tube surface.Therefore, the shorter length tubes (corresponding to a shorter portseparation distance S) are more efficient. On the other hand, a shorterseparation distance S affords less opportunity for plasma current flowseparated at its center by the triangular splitter 5510 to reenter thecenter region after passing the splitter 5510 and avoid a low plasma iondensity at the wafer center. Thus, there would appear to be a tradeoffbetween the higher efficiency of a smaller port separation distance Sand the risk of depressing plasma ion density at the wafer center in theeffort to avoid a plasma hot spot near each reentrant tube port.

[0212] This tradeoff is ameliorated or eliminated in the case of FIGS.65A, 65B and 66, by using a triangular splitter 6510 that extends atleast nearly across the entire width W of the termination section 5520of the port and is shaped to force plasma current flow away from theinner edge 6610 of the port and toward the outer edge 6620 of the port.This feature leaves the port separation distance S unchanged (so that itmay be as short as desired), but in effect lengthens the plasma currentpath from the apex 6510 a of the splitter to the center of the wafer120. This affords a greater opportunity for the plasma current flowsplit by the splitter 6510 to rejoin at its center before reaching thewafer or center of the wafer. This feature better avoids depressingplasma ion density at the wafer center while suppressing formation ofplasma hot spots at the reentrant tube ports.

[0213] As illustrated in FIGS. 65A, 65B and 66, each splitter 6510presents an isoceles triangular shape in elevation (FIG. 65B) and arectangular shape from the top (FIG. 65A). The side view of FIG. 66reveals the sloping back surface 6610 c that extends downwardly towardthe outer edge 6620 of the port. It is the sloping back surface 6610 cthat forces the plasma current toward the back edge 6620 therebyeffectively lengthening the path from the top of the apex 6510 a to thewafer center, which is the desired feature as set forth above. Therectangular opening of the port 150 is narrowed in the radial direction(the short dimension) by the sloped wall or sloping back surface 6610 bfrom about 2″ at top to about ¾″ at the bottom. This pushes the innerport edge about 1¼ radially farther from the wafer (thus achieving thedesired increase in effective port separation distance). In addition,the port 150 has the full triangular splitter 6510 in the azimuthaldirection (the long or 8″ wide dimension of the opening 150).

[0214] The plasma current splitter 5510 or 6510 may have coolantpassages extending within it with coolant ports coupled to similar portsin the reactor body to regulate the temperature of the splitter. Forthis purpose, the plasma current splitter 5510 or 6510 is formed ofmetal, since it easily cooled and is readily machined to form internalcoolant passages. However, the splitter 5510 or 6510 may instead beformed of another material such as quartz, for example.

[0215]FIG. 67 illustrates another way of improving plasma uniformity inthe torroidal source reactor of FIG. 24 by introducing a set of fourannular electromagnets 6710, 6720, 6730, 6740 along the periphery of thereactor, the windings of each electromagnet being controlled by a magnetcurrent controller 6750. The electric currents in the fourelectromagnets may be driven in any one of three modes:

[0216] (1) in a first mode, a sinusoidal mode, the coils are driven atthe same low frequency current in phase quadrature to produce a magneticfield that rotates about the axis of symmetry of the reactor at the lowfrequency of the source;

[0217] (2) in a second mode, a configurable magnetic field mode, thefour electromagnets 6710, 6720, 6730, 6740 are grouped into opposingpairs of adjacent electromagnets, and each pair is driven with adifferent D.C. current to produce a magnetic field gradient extendingdiagonally between the opposing pairs of adjacent electromagnets, andthis grouping is rotated so that the magnetic field gradient is rotatedto isotropically distribute its effects over the wafer;

[0218] (3) in a third mode, the four electromagnets are all driven withthe same D.C. current to produce a cusp-shaped magnetic field having anaxis of symmetry coinciding generally with the axis of symmetry of thereactor chamber.

[0219] As shown in FIG. 1, a pumping annulus is formed between thecylindrical wafer support pedestal 115 and the cylindrical side wall105, gases being evacuated via the pumping annulus by the vacuum pump135. Plasma current flow between the opposing ports of each reentranttube 150 can flow through this pumping annulus and thereby avoid flowingthrough the processing region between the wafer 120 and the gasdistribution plate 210. Such diversion of plasma current flow around theprocess region can occur if the chamber pressure is relatively high andthe wafer-to-ceiling gap is relatively small and/or the conductivity ofthe plasma is relatively low. To the extent this occurs, plasma iondensity in the process region is reduced. This problem is solved asshown in FIGS. 68 and 69 by the introduction of radial vanes 6910, 6920,6930, 6940 blocking azimuthal plasma current flow through the pumpingannulus. In one implementation, the vanes 6910, 6920, 6930, 6940 extendup to but not above the plane of the wafer 120, to allow insertion andremoval of the wafer 120. However, in another implementation the vanesmay retractably extend above the plane of the wafer to better confinethe plasma current flow within the processing region overlying the wafer120. This may be accomplished by enabling the wafer support pedestal 115to move up and down relative to the vanes, for example. In either case,the vanes 6910, 6920, 6930, 6940 prevent plasma current flow through thepumping annulus, and, if the vanes can be moved above the plane of thewafer 120, they also reduce plasma current flow through the upper regionoverlying the pumping annulus. By thus preventing diversion of plasmacurrent flow away from the processing region overlying the wafer, notonly is plasma ion density improved in that region but process stabilityis also improved.

[0220] As mentioned previously herein, the magnetic core used to coupleRF power to each reentrant tube 150 tends to crack or shatter at high RFpower levels. It is believed this problem arises because magnetic fluxis not distributed uniformly around the core. Generally, one windingaround the core has a high current at high RF power levels. This windingcan be, for example, a secondary winding that resonates the primarywinding connected to the RF generator. The secondary winding isgenerally confined to a narrow band around the core, magnetic flux andheating being very high within this band and much lower elsewhere in thecore. The magnetic core must have a suitable permeability (e.g., apermeability between about 10 and 200) to avoid self-resonance at highfrequencies. A good magnetic core tends to be a poor heat conductor (lowthermal conductivity) and be readily heated (high specific heat), and istherefore susceptible to localized heating. Since the heating islocalized near the high current secondary winding and since the coretends to be brittle, it cracks or shatters at high RF power levels(e.g., 5 kilowatts of continuous power).

[0221] This problem is solved in the manner illustrated in FIGS. 70through 74 by more uniformly distributing RF magnetic flux densityaround the annular core. FIG. 70 illustrates a typical one of themagnetic cores 1015 of FIG. 17A. The core 1015 is formed of a highmagnetic permeability material such as ferrite. The primary winding 170consists of about two turns of a thin copper band optionally connectedthrough an impedance match device 175 to the RF generator 180. Highcurrent flow required for high magnetic flux in the core 1015 occurs ina resonant secondary winding 7010 around the core 1015. Current flow inthe secondary winding 7010 is about an order of magnitude greater thancurrent flow in the primary winding. In order to uniformly distributemagnetic flux around the core 1015, the secondary winding 7010 isdivided into plural sections 7010 a, 7010 b, 7010 c, etc., that areevenly distributed around the annular core 1015. The secondary windingsections 7010 a, etc., are connected in parallel. Such parallelconnection is facilitated as illustrated in FIGS. 71A and 71B by a pairof circular copper buses 7110, 7120 extending around opposite sides ofthe magnetic core 1015. Opposing ends of each of the secondary windings7010 a, 7010 b, etc., are connected to opposite ones of the two copperbuses 7110, 7120. The copper buses 7110, 7120 are sufficiently thick toprovide an extremely high conductance and low inductance, so that theazimuthal location of any particular one of the secondary windingsections 7010 a, 7010 b, etc. makes little or no difference, so that allsecondary winding sections function as if they were equidistant from theprimary winding. In this way, magnetic coupling is uniformly distributedaround the entire core 1015.

[0222] Because of the uniform distribution of magnetic flux achieved bythe foregoing features, the primary winding may be placed at anysuitable location, typically near a selected one of the pluraldistributed secondary winding sections 7110 a, 7110 b, 7110 c, etc.However, in one implementation, the primary winding is wrapped around oron a selected one of the plural distributed secondary winding sections7110 a, 7110 b, 7110 c, etc.

[0223]FIG. 72 is a representation of the distributed parallelinductances formed by the parallel secondary winding sections 7010 a,7010 b, etc., and FIG. 73 shows the circular topology of thesedistributed inductances. In order to provide resonance at the frequencyof the RF generator 180, plural distributed capacitors 7130 areconnected in parallel across the two copper buses 7110, 7120. The pluralcapacitors 7030 are distributed azimuthally around the magnetic core1015. Each capacitor 7030 in one implementation was about 100picoFarads. The equivalent circuit of the distributed inductances andcapacitances associated with the secondary winding 7010 is illustratedin FIG. 24.

[0224] Referring to FIG. 71B, the secondary winding sections 7010 a,7010 b, etc., can have the same number of turns. In the case of FIG.71B, there are six secondary winding sections 7010 a-7010 f, eachsection having three windings. The skilled worker can readily select thenumber of secondary winding sections, the number of windings in eachsection and the capacitance of the distributed capacitors 7030 toachieve resonance at the frequency of the RF generator 180. The copperband stock used to form the primary and secondary windings around thecore 1015 can be, for example, 0.5 inch wide and 0.020 inch thick copperstripping. The two copper buses 7110, 7120 are very thick (e.g., from0.125 inch to 0.25 inch thick) and wide (e.g., 0.5 inch wide) so thatthey form extremely low resistance, low inductance current paths. Thecore 1015 may consist of a pair of stacked 1 inch thick ferrite coreswith a 10 inch outer diameter and an 8 inch inner diameter. Preferably,the ferrite core 1015 has a magnetic permeability μ=40. The foregoingdetails are provided by way of example only, and any or all of theforegoing values may require modification for different applications(e.g., where, for example, the frequency of the RF generator ismodified).

[0225] We have found that the feature of distributed inductancesillustrated in FIGS. 71A and 71B solves the problem of breakage of themagnetic core experienced at sustained high RF power levels (e.g., 5kilowatts).

[0226]FIG. 75 illustrates the equivalent circuit formed by the core andwindings of FIGS. 71A and 71B. In addition to the primary and secondarywindings 170 and 7010 around the core 1015, FIG. 75 illustrates theequivalent inductive and capacitive load presented by the plasmainductively coupled to the core 1015. The case of FIGS. 70-75 is atransformer coupled circuit. The purpose of the secondary winding 7010is to provide high electric current flow around the magnetic core 1015for enhanced power coupling via the core. The secondary winding 7010achieves this by resonating at the frequency of the RF generator. Thus,the high current flow and power coupling via the magnetic core 1015occurs in the secondary winding 7010, so that virtually all the heatingof the core 1015 occurs at the secondary winding 7010. By thusdistributing the secondary winding 7010 around the entire circumferenceof the core 1015, this heating is similarly distributed around the coreto avoid localized heating and thereby prevent shattering the core athigh RF power levels.

[0227] The distributed winding feature of FIGS. 71A and 71B can be usedto implement other circuit topologies, such as the auto transformercircuit of FIG. 76. In the auto transformer circuit of FIG. 76, thewinding 7010 around the core 1015 is distributed (in the mannerdiscussed above with reference to FIGS. 70-74) and has a tap 7610connected through the impedance match circuit 175 to the RF generator180. The distributed capacitors 7030 provide resonance (in the mannerdiscussed above). As in FIG. 70, the core 7010 is wrapped around thereentrant tube 150 so that power is inductively coupled into theinterior of the tube 150. The circuit topologies of FIGS. 75 and 76 areonly two examples of the various topologies that can employ distributedwindings around the magnetic core 1015.

[0228] In one implementation, the impedance match circuits 175 a, 175 bemployed frequency tuning in which the frequency of each RF generator180 a, 180 b is controlled in a feedback circuit in such a way as tominimize reflected power and maximize forward or delivered power. Insuch an implementation, the frequency tuning ranges of each of thegenerators 180 a, 180 b are exclusive, so that their frequencies alwaysdiffer, typically on the order of a 0.2 to 2 MHz difference. Moreover,their phase relationship is random. This frequency difference canimprove stability. For example, instabilities can arise if the samefrequency is used to excite plasma in both of the orthogonal tubes150-1, 150-2. Such instabilities can cause the plasma current to flowthrough only three of the four ports 155, 160, for example. Thisinstability may be related to the phase difference between the torroidalplasma currents in the tubes. One factor facilitating plasma stabilityis isolation between the two plasma currents of the pair of orthogonaltubes 150-1, 150-2. This isolation is provided mainly by the plasmasheaths of the two plasma currents. The D.C. break or gap 152 of each ofthe reentrant tubes 150-1, 150-2 also enhances plasma stability.

[0229] While the D.C. break or gap 152 in each of the orthogonal tubesis illustrated in FIG. 44 as being well-above the chamber ceiling 110,it may in fact be very close to or adjacent the ceiling. Such anarrangement is employed in the implementation of FIG. 77, in which thecase of FIG. 55A is modified so that the termination section 5520electrically floats so that its potential follows oscillations of theplasma potential. This solves a problem that can be referred to as a“hollow cathode” effect near each of the ports 155, 160 that createsnon-uniform plasma distribution. This effect may be referred to as anelectron multiplication cavity effect. By permitting all of theconductive material near a port to follow the plasma potentialoscillations, the hollow cathode effects are reduced or substantiallyeliminated. This is achieved by electrically isolating the terminationsection 5520 from the grounded chamber body by locating a D.C. break orgap 152′ at the juncture between the reentrant tube termination section5520 and the top or external surface of the ceiling 110. (The gap 152′may be in addition to or in lieu of the gap 152 of FIG. 44.) The gap152′ is filled with an insulative annular ring 7710, and the terminationsection 5520 of FIG. 77 has a shoulder 7720 resting on the top of theinsulative ring 7710. Moreover, there is an annular vacuum gap 7730 ofabout 0.3 to 3 mm width between the ceiling 110 and the terminationsection 5520. In one implementation, the tube 150 and the terminationsection 5520 are integrally formed together as a single piece. Thetermination section 5520 is preferably formed of metal so that internalcoolant passages may be formed therein.

[0230] FIGS. 44-77 illustrate cases in which the uniformity controlmagnet is above the processing region. FIG. 78 illustrates that themagnet pole 4440 may be placed below the processing region, or under thewafer support pedestal 115.

WORKING EXAMPLES

[0231] An etch process was conducted on blanket oxide wafers at achamber pressure of 40 mT, 4800 watts of 13.56 MHz RF bias power on thewafer pedestal and 1800 Watts of RF source power applied to eachreentrant tube 150 at 11.5 MHz and 12.5 MHz, respectively. The magneticfield produced by the electromagnet assembly 4430 was set at thefollowing levels in successive steps: (a) zero, (b) 6 Gauss and (c) 18Gauss (where the more easily measured axial magnetic field component atthe wafer center was observed rather than the more relevant radialcomponent). The observed etch rate distribution on the wafer surface wasmeasured, respectively, as (a) center low with a standard deviation ofabout 2% at zero Gauss, (b) slightly center fast with a standarddeviation of about 1.2% at 6 Gauss, and (c) center fast with a standarddeviation of 1.4%. These examples demonstrate the ability to providenearly ideal compensation (step b) and the power to overcompensate (stepc).

[0232] To test the effective pressure range, the chamber pressure wasincreased to 160 mT and the electromagnet's field was increased in threesteps from (a) zero Gauss, to (b) 28 Gauss and finally to (c) 35 Gauss(where the more easily measured axial magnetic field component at thewafer center was observed rather than the more relevant radialcomponent). The observed etch rate was, respectively, (a) center slowwith a standard deviation of about 2.4%, center fast with a standarddeviation of about 2.9% and center fast with a standard deviation ofabout 3.3%. Obviously, the step from zero to 28 Gauss resulted inovercompensation, so that a somewhat smaller magnetic field would havebeen ideal, while the entire exercise demonstrated the ability of theelectromagnet assembly 4430 to easily handle very high chamber pressureranges. This test was severe because at higher chamber pressures theetch rate distribution tends to be more severely center low while, atthe same time, the decreased collision distance or mean free path lengthof the higher chamber pressure makes it more difficult for a givenmagnetic field to effect plasma electrons or ions. This is because themagnetic field can have no effect at all unless the correspondingLarmour radius of the plasma electrons or ions (determined by thestrength of the magnetic field and the mass of the electron or ion) doesnot exceed the plasma collision distance. As the collision distancedecreases with increasing pressure, the magnetic field strength must beincreased to reduce the Larmour radius proportionately. The foregoingexamples demonstrate the power of the electromagnet assembly to generatea sufficiently strong magnetic field to meet the requirement of a smallLarmour radius.

[0233] Another set of etch processes were carried out on oxide waferspatterned with photoresist at 35 mT under similar conditions, and thecurrent applied to the electromagnet assembly 4430 was increased in fivesteps from (a) 0 amperes, (b) 5 amperes, (c) 6 amperes, (d) 7 amperesand (e) 8 amperes. (In this test, a current of 5 amperes produces about6 gauss measured axial magnetic field component at the wafer center.) Ateach step, the etch depths of high aspect ratio contact openings weremeasured at both the wafer center and the wafer periphery to testcenter-to-edge etch rate uniformity control. The measured center-to-edgeetch rate differences were, respectively, (a) 13.9% center low, (b) 3.3%center low, (c) 0.3% center low, (d) 2.6% center high and (e) 16.3%center high. From the foregoing, it is seen that the ideal electromagnetcurrent for best center-to-edge uniformity is readily ascertained and inthis case was about 6 amperes.

[0234] A set of etch processes were carried out on blanket oxide wafersto test the efficacy of the dual zone gas distribution plate 210 of FIG.44. In a first step, the gas flow rates through the two zones wereequal, in a second step the inner zone had a gas flow rate four timesthat of the outer zone and in a third step the outer zone had a gas flowrate four times that of the inner zone. In each of these steps, nocurrent was applied to the electromagnet assembly 4430 so that themeasurements taken would reflect only the effect of the dual zone gasdistribution plate 210. With gas flow rates of the two zones equal inthe first step, the etch rate distribution was slightly center high witha standard deviation of about 2.3%. With the inner zone gas flow rate atfour times that of the outer zone, the etch rate distribution was centerfast with a standard deviation of about 4%. With the outer zone gas flowrate at four times that of the inner zone, the etch rate distributionwas center slow with a standard deviation of about 3.4%. This showedthat the dual zone differential gas flow rate feature of the gasdistribution plate 210 can be used to make some correction to the etchrate distribution. However, the gas flow rate control directly affectsneutral species distribution only, since none of the incoming gas is (orshould be) ionized. On the other hand, etch rate is directly affected byplasma ion distribution and is not as strongly affected by neutraldistribution, at least not directly. Therefore, the etch ratedistribution control afforded by the dual zone gas distribution plate,while exhibiting some effect, is necessarily less effective than themagnetic confinement of the electromagnet assembly 4430 which directlyaffects plasma electrons and thus ions.

[0235] The dependency of the electromagnet assembly 4430 upon thereentrant torroidal plasma current was explored. First a series of etchprocesses was carried out on blanket oxide wafers with no power appliedto the torroidal plasma source, the only power being 3 kilowatts of RFbias power applied to the wafer pedestal. The electromagnet coil currentwas increased in four steps of (a) zero amperes, (b) 4 amperes, (c) 6amperes and (d) 10 amperes. The etch rate distribution was observed inthe foregoing steps as (a) center high with a standard deviation of2.87%, (b) center high with a standard deviation of 3.27%, (c) centerhigh with a standard deviation of 2.93% and (d) center high with astandard deviation of about 4%. Thus, only a small improvement inuniformity was realized for a relatively high D.C. current applied tothe electromagnet assembly 4430. Next, a series of etch processes wascarried out under similar conditions, except that 1800 Watts was appliedto each of the orthogonal tubes 150-1, 150-2. The electromagnet coilcurrent was increased in six steps of (a) zero amperes, (b) 2 amperes,(c) 3 amperes, (d) 4 amperes, (e) 5 amperes and (f) 6 amperes. The etchrate distribution was, respectively, (a) center low with a standarddeviation of 1.2%, (b) center low with a standard deviation of 1.56%,(c) center high with a standard deviation of 1.73%, (d) center high witha standard deviation of 2.2%, (e) center high with a standard deviationof 2.85% and (f) center high with a standard deviation of 4.25%.Obviously the most uniform distribution lies somewhere between 2 and 3amperes where the transition from center low to center high was made.Far greater changes in plasma distribution were made using much smallercoil current with much smaller changes in coil current. Thus, thepresence of the reentrant torroidal plasma currents appears to enhancethe effects of the magnetic field of the electromagnet assembly 4430.Such enhancement may extend from the increase in bias power that ispossible when the torroidal plasma source is activated. In its absence,the plasma is less conductive and the plasma sheath is much thicker, sothat the bias RF power applied to the wafer pedestal must necessarily belimited. When the torroidal plasma source is activated (e.g., at 1800Watts for each of the two orthogonal tubes 150-1, 150-2) the plasma ismore conductive, the plasma sheath is thinner and more bias power can beapplied. As stated before herein, the effect of the D.C. magnetic fieldmay be dependent upon the interaction between the D.C. magnetic fieldand the electric field of the plasma sheath, which in turn depends uponthe RF bias power applied to the pedestal. Furthermore, the reentranttorroidal plasma currents may be attracted to the central plasma regiondue to the aforementioned postulated interaction between D.C. magneticfield and the electric field of the plasma sheath, further enhancing theplasma ion density in that region.

[0236] The effects of the port-to-port separation distance S of FIG. 55Awere explored in another series of etch processes on blanket oxidewafers. The same etch process was carried out in reactors havingseparation distances S of 16.5 inches and 20.5 inches respectively. Theetch rate in the one with smaller separation distance was 31% greaterthan in the one with the greater separation distance (i.e., 6993 vs 5332Angstroms/minute) with 1800 Watts applied to each one of the orthogonaltubes 150-1, 150-2 with zero current applied to the electromagnetassembly 4300 in each reactor.

[0237] The effects of the port-to-port separation distance S of FIGS.55-56 were also explored in another series of etch processes on oxidewafers patterned with photoresist. With 3.7 amperes applied to theelectromagnet assembly 4300 having the smaller source (16.5 inch)separation distance S, the etch rate was 10450 Angstroms/minute vs 7858Angstroms/minute using the larger source (20.5 inch) separation distanceS. The effect of increasing power in the reactor having the greater(20.5 inches) separation distance S was explored. Specifically, the sameetch process was carried out in that reactor with source power appliedto each of the orthogonal tubes 150-1, 150-2 being 1800 Watts and thenat 2700 Watts. The etch rate increased proportionately very little,i.e., from 7858 Angstroms/minute to 8520 Angstroms/minute. Thus, theeffect of the port-to-port separation distance S on plasma ion densityand etch rate cannot readily be compensated by changing plasma sourcepower. This illustrates the importance of cases such as the case ofFIGS. 65A, 65B and 66 in which a relatively short port-to-portseparation distance S is accommodated while in effect lengthening thedistance over which the plasma current is permitted to equilibrate afterbeing split by the triangular splitters 5440.

[0238] The pole piece 4440 has been disclosed a being either a permanentmagnet or the core of an electromagnet surrounded by a coil 4450.However, the pole piece 4440 may be eliminated, leaving only the coil4450 as an air coil inductor that produces a magnetic field having asimilar orientation to that produced by the pole piece 4440. The aircoil inductor 4450 may thus replace the pole piece 4440. Therefore, inmore general terms, what is required to produce the requisite radialmagnetic field is an elongate pole-defining member which may be eitherthe pole piece 4440 or an air coil inductor 4450 without the pole piece4440 or the combination of the two. The diameter of the pole-definingmember is relatively narrow to appropriately confine the peak of theradial magnetic field.

Plasma Immersion Ion Implantation

[0239] Referring to FIG. 79, a plasma immersion ion implantation reactorin accordance with one aspect of the invention includes a vacuum chamber8010 having a ceiling 8015 supported on an annular side wall 8020. Awafer support pedestal 8025 supports a semiconductor (e.g., silicon)wafer or workpiece 8030. A vacuum pump 8035 is coupled to a pumpingannulus 8040 defined between the pedestal 8025 and the side wall 8020. Abutterfly valve 8037 regulates gas flow into the intake of the pump 8035and controls the chamber pressure. A gas supply 8045 furnishes processgas containing a dopant impurity into the chamber 8010 via a system ofgas injection ports that includes the injection port 8048 shown in thedrawing. For example, if the wafer 8030 is a crystalline silicon wafer aportion of which is to be implanted with a p-type conductivity dopantimpurity, then the gas supply 8045 may furnish BF₃ and/or B₂H₆ gas intothe chamber 8010, where Boron is the dopant impurity species. Generally,the dopant-containing gas is a chemical consisting of the dopantimpurity, such as boron (a p-type conductivity impurity in silicon) orphosphorus (an n-type conductivity impurity in silicon) and a volatilespecies such as fluorine and/or hydrogen. Thus, fluorides and/orhydrides of boron, phosphorous or other dopant species such as arsenic,antimony, etc., can be dopant gases. In a plasma containing a fluorideand/or hydride of a dopant gas such as BF₃, there is a distribution ofvarious ion species, such as BF₂+, BF+, B+, F+, F− and others (such asinert additives). All types of species may be accelerated across thesheath and may implant into the wafer surface. The dopant atoms (e.g.,boron or phosphorous atoms) typically dissociate from the volatilespecies atoms (e.g., fluorine or hydrogen atoms) upon impact with thewafer at sufficiently high energy. Although both the dopant ions andvolatile species ions are accelerated into the wafer surface, someportion of the volatile species atoms tend to leave the wafer during theannealing process that follows the ion implantation step, leaving thedopant atoms implanted in the wafer.

[0240] A plasma is generated from the dopant-containing gas within thechamber 8010 by an inductive RF power applicator including an overheadcoil antenna 8050 coupled to an RF plasma source power generator 8055through an impedance match circuit 8060. An RF bias voltage is appliedto the wafer 8030 by an RF plasma bias power generator 8065 coupled tothe wafer support pedestal 8025 through an impedance match circuit 8070.A radially outer coil antenna 8052 may be driven independently by asecond RF plasma source power generator 8057 through an impedance matchcircuit 8062.

[0241] The RF bias voltage on the wafer 8030 accelerates ions from theplasma across the plasma sheath and into the wafer surface, where theyare lodged in generally interstitial sites in the wafer crystalstructure. The ion energy, ion mass, ion flux density and total dose maybe sufficient to amorphize (damage) the structure of the wafer. The massand kinetic energy of the dopant (e.g., boron) ions at the wafer surfaceand the structure of the surface itself determine the depth of thedopant ions below the wafer surface. This is controlled by the magnitudeof the RF bias voltage applied to the wafer support pedestal 8025. Afterthe ion implantation process is carried out, the wafer is subjected toan anneal process that causes the implanted dopant atoms to move intosubstitutional atomic sites in the wafer crystal. The substrate surfacemay not be crystalline if it has been pre-amorphized prior to the plasmaimmersion ion implant process, or if the ion energy, ion mass, ion fluxdensity and total dose of plasma immersion ion implant process itself issufficient to amorphize the structure of the wafer. In such a case, theanneal process causes the amorphous (damaged) layer to recrystallizewith the incorporation and activation of implanted dopant. Theconductance of the implanted region of the semiconductor is determinedby the junction depth and the volume concentration of the activatedimplanted dopant species after the subsequent anneal process. If, forexample, a p-type conductivity dopant such as boron is implanted into asilicon crystal which has been previously doped with an n-type dopantimpurity, then a p-n junction is formed along the boundaries of thenewly implanted p-type conductivity region, the depth of the p-njunction being the activated implanted depth of the p-type dopantimpurities after anneal. The junction depth is determined by the biasvoltage on the wafer (and by the anneal process), which is controlled bythe power level of the RF plasma bias power generator 8065. The dopantconcentration in the implanted region is determined by the dopant ionflux (“dose”) at the wafer surface during implantation and the durationof the ion flux. The dopant ion flux is determined by the magnitude ofthe RF power radiated by the inductive RF power applicator 8050, whichis controlled by the RF plasma source power generator 8055. Thisarrangement enables independent control of the time of implant, theconductivity of the implanted region and the junction depth. Generally,the control parameters such as the power output levels of the bias powerRF generator 8065 and the source power RF generator 8055 are chosen tominimize the implant time while meeting the target values forconductivity and junction depth. For more direct control of ion energy,the bias generator may have “voltage” rather than “power” as its outputcontrol variable.

[0242] An advantage of the inductive RF plasma source power applicator8050 is that the ion flux (the dopant dose rate) can be increased byincreasing the power level of the RF source power generator 8055 withouta concomitant increase in plasma potential. The bias voltage level iscontrolled by the RF bias power generator at a preselected value(selected for the desired implant depth) while the inductive RF sourcepower is increased to increase the ion flux (the dopant dose rate)without significantly increasing the plasma potential. This featureminimizes contamination due to sputtering or etching of chambersurfaces. It further reduces the consumption of consumable componentswithin the chamber that wear out over time due to plasma sputtering.Since the plasma potential is not necessarily increased with ion flux,the minimum implant energy is not limited (increased), thereby allowingthe user to select a shallower junction depth than would otherwise havebeen possible. In contrast, it will be recalled that the microwave ECRplasma source was characterized by a relatively high minimum plasmapotential, which therefore limited the minimum implant energy andtherefore limited the minimum junction depth.

[0243] An advantage of applying an RF bias voltage to the wafer (insteadof a D.C. bias voltage) is that it is far more efficient (and thereforemore productive) for ion implantation, provided the RF bias frequency issuitably chosen. This is illustrated in FIGS. 80A, 80B and 80C. FIG. 80Aillustrates a one-millisecond D.C. pulse applied to the wafer inconventional practice, while FIG. 80B illustrates the resulting ionenergy at the wafer surface. The D.C. pulse voltage of FIG. 80A is nearthe target bias voltage at which ions become substitutional uponannealing at the desired implant junction depth. FIG. 80B shows how theion energy decays from the initial value corresponding to the voltage ofthe pulse of FIG. 80A, due to resistive-capacitive effects at the wafersurface. As a result, only about the first micro-second (or less) of theone-millisecond D.C. pulse of FIG. 80A is actually useful, because it isonly this micro-second portion of the pulse that produces ion energiescapable of implanting ions that become substitutional (during annealing)at the desired junction depth. The initial (one microsecond) period ofthe D.C. pulse may be referred to as the RC time. During the remainingportion of the D.C. pulse, ions fail to attain sufficient energy toreach the desired depth or to become substitutional upon annealing, andmay fail to penetrate the wafer surface so as to accumulate in adeposited film that resists further implantation. This problem cannot besolved by increasing the pulse voltage, since this would produce a largenumber of ions that would be implanted deeper than the desired junctiondepth. Thus, ions are implanted down to the desired junction depthduring only about a tenth of a percent of the time. This increases thetime required to reach the target implant density at the desiredjunction depth. The resulting spread in energy also reduces theabruptness of the junction. In contrast, each RF cycle in a 1millisecond burst of a 1 MHz RF bias voltage illustrated in FIG. 80C hasan RF cycle time not exceeding the so-called RC time of FIG. 80B. As aresult, resistive-capacitive effects encountered with a pulsed D.C. biasvoltage are generally avoided with an RF bias voltage of a sufficientfrequency. Therefore, ions are implanted down to the desired junctiondepth during a far greater percentage of the time of the 1 MHz RF biasvoltage of FIG. 80C. This reduces the amount of time required to reach atarget implant density at the desired junction depth. Thus, the use ofan RF bias voltage on the wafer results in far greater efficiency andproductivity than a D.C. pulse voltage, depending upon the choice of RFfrequency.

[0244] The frequency of the RF bias is chosen to satisfy the followingcriteria: The RF bias frequency must be sufficiently high to have anegligible voltage drop across the pedestal (cathode) dielectric layers)and minimize sensitivity to dielectric films on the backside or frontside of the wafer and minimize sensitivity to chamber wall surfaceconditions or deposition of plasma by-products. Moreover, the frequencymust be sufficiently high to have a cycle time not significantlyexceeding the initial period (e.g., one micro-second) beforeresistive-capacitive (RC) effects reduce ion energy more than 2% belowthe target energy, as discussed immediately above. Furthermore, the RFbias frequency must be sufficiently high to couple across insulatingcapacitances such as films on the wafer surface, dielectric layers onthe wafer support pedestal, coatings on the chamber walls, or depositedfilms on the chamber walls. (An advantage of RF coupling of the biasvoltage to the wafer is that such coupling does not rely upon ohmiccontact and is less affected by changes or variations in the surfaceconditions existing between the wafer and the support pedestal.)However, the RF bias frequency should be sufficiently low so as to notgenerate significant plasma sheath oscillations (leaving that task tothe plasma source power applicator). More importantly, the RF biasfrequency should be sufficiently low for the ions to respond to theoscillations of the electric field in the plasma sheath overlying thewafer surface. The considerations underlying this last requirement arenow discussed with reference to FIGS. 81A through 81D.

[0245]FIG. 81A illustrates the plasma ion saturation current at thewafer surface as a function of D.C. bias voltage applied to the wafer,the current being greatest (skewed toward) the higher voltage region.FIG. 81B illustrates the oscillation of the RF voltage of FIG. 80C. Theasymmetry of the ion saturation current illustrated in FIG. 80A causesthe ion energy distribution created by the RF bias voltage of FIG. 80Bto be skewed in like manner toward the higher energy region, asillustrated in FIG. 80C. The ion energy distribution is concentratedmost around an energy corresponding to the peak-to-peak voltage of theRF bias on the wafer. But this is true only if the RF bias frequency issufficiently low for ions to follow the oscillations of the electricfield in the plasma sheath. This frequency is generally a low frequencyaround 100 kHz to 3 MHz, but depends on sheath thickness andcharge-to-mass ratio of the ion. Sheath thickness is a function ofplasma electron density at the sheath edge and sheath voltage. Referringto FIG. 81D, as this frequency is increased from the low frequency(denoted F1 in FIG. 81D) to a medium frequency (denoted F2 in FIG. 81D)and finally to a high frequency such as 13 MHz (denoted F3 in FIG. 81D),the ability of the ions to follow the plasma sheath electric fieldoscillation is diminished, so that the energy distribution is narrower.At the HF frequency (F3) of FIG. 81D, the ions do not follow the sheathelectric field oscillations, and instead achieve an energy correspondingto the average voltage of the RF bias voltage, i.e., about half the RFbias peak-to-peak voltage. As a result, the ion energy is cut in half asthe RF bias frequency increases to an HF frequency (for a constant RFbias voltage). Furthermore, at the medium frequency, we have found thatthe plasma behavior is unstable in that it changes sporadically betweenthe low frequency behavior (at which the ions have an energycorresponding to the peak-to-peak RF bias voltage) and the highfrequency behavior (at which the ions have an energy corresponding toabout half the peak-to-peak RF bias voltage). Therefore, by maintainingthe RF bias frequency at a frequency that is sufficiently low(corresponding to the frequency F1 of FIG. 81D) for the ions to followthe plasma sheath electric field oscillations, the RF bias peak-to-peakvoltage required to meet a particular ion implant depth requirement isreduced by a factor of nearly two, relative to behavior at a mediumfrequency (F2) or a high frequency (F3). This is a significant advantagebecause such a reduction in the required RF bias voltage (e.g., by afactor of two) greatly reduces the risk of high voltage arcing in thewafer support pedestal and the risk of damaging thin film structures onthe wafer. This is particularly important because in at least aparticular plasma immersion ion implantation source described later inthis specification, ion energies match those obtained in a conventionalion beam implanter, provided the plasma RF bias voltage is twice theacceleration voltage of the conventional ion beam implanter. Thus, at ahigh frequency plasma RF bias voltage, where ion energies tend to behalf those obtained at low frequency, the required plasma RF biasvoltage is four times the acceleration voltage of the conventional ionbeam implanter for a given ion energy level. Therefore, it is importantin a plasma immersion ion implantation reactor to exploit the advantagesof a low frequency RF bias voltage, to avoid the necessity of excessiveRF bias voltages.

[0246] Good results are therefore attained by restricting the RF biaspower frequency to a low frequency range between 10 kHz and 10 MHz.Better results are obtained by limiting the RF bias power frequency to anarrower range of 50 kHz to 5 MHz. The best results are obtained in theeven narrower bias power frequency range of 100 kHz to 3 MHz. We havefound optimum results at about 2 MHz plus or minus 5%.

[0247] Both the RF source power generator 8055 and the RF bias powergenerator 8065 may apply continuous RF power to the inductive powerapplicator 8050 and the wafer pedestal 8025 respectively. However,either or both of the generators 8055, 8065 may be operated in burstmodes controlled by a controller 8075. The controller 8075 may alsocontrol the generator 8057 in a burst mode as well if the outer coilantenna 8052 is present. Operation in an implementation not includingthe outer coil antenna 8057 will now be described. The RF signalsproduced by each of the generators 8055, 8065 may be pulse modulated toproduce continuous wave (CW) RF power in bursts lasting, for example,one millisecond with a repetition rate on the order of 0.5 kHz, forexample. Either one or both of the RF power generators 8055, 8065 may beoperated in this manner. If both are operated in such a burst modesimultaneously, then they may be operated in a push-pull mode, or in anin-synchronism mode, or in a symmetrical mode or in a non-symmetricalmode, as will now be described.

[0248] A push-pull mode is illustrated in the contemporaneous timedomain waveforms of FIGS. 82A and 82B, illustrating the RF powerwaveforms of the respective RF generators 8055 and 8065, in which thebursts of RF energy from the two generators 8055, 8065 occur duringalternate time windows. FIGS. 82A and 82B illustrate the RF powerwaveforms of the generators 8055 and 8065, respectively, or vice versa.

[0249] An in-synchronism mode is illustrated in the contemporaneous timedomain waveforms of FIGS. 82C and 82D, in which the bursts of RF energyfrom the two generators 8055, 8065 are simultaneous. They may not benecessarily in phase, however, particularly where the two generators8055, 8065 produce different RF frequencies. For example, the RF plasmasource power generator 8055 may have a frequency of about 13 MHz whilethe RF plasma bias power generator 8065 may have a frequency of about 2MHz. FIGS. 82C and 82D illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

[0250] In the foregoing examples, the pulse widths and pulse repetitionrates of the two RF generators 8055, 8065 may be at least nearly thesame. However, if they are different, then the temporal relationshipbetween the bursts of the two generators 8055, 8065 must be selected. Inthe example of the contemporaneous time domain waveforms of FIGS. 82Eand 82F, one of the generators 8055, 8065 produces shorter RF burstsillustrated in FIG. 82F while the other produces longer RF burstsillustrated in FIG. 82E. In this example, the bursts of the twogenerators 8055, 8065 are symmetrically arranged, with the shorterbursts of FIG. 82F centered with respect to the corresponding longerbursts of FIG. 82E. FIGS. 82E and 82F illustrate the RF power waveformsof the generators 8055 and 8065, respectively, or vice versa.

[0251] In another example, illustrated in the contemporaneous timedomain waveforms of FIGS. 82G and 82H, the shorter bursts (FIG. 82H) arenot centered relative to the corresponding longer bursts (FIG. 82G), sothat they are asymmetrically arranged. Specifically, in this example theshorter RF bursts of FIG. 82H coincide with the later portions ofcorresponding ones of the long bursts of FIG. 82G. Alternatively, asindicated in dashed line in FIG. 82H, the short RF bursts of FIG. 82Hmay instead coincide with the earlier portions of corresponding ones ofthe long RF bursts of FIG. 82G. FIGS. 82G and 82H illustrate the RFpower waveforms of the generators 8055 and 8065, respectively, or viceversa.

[0252] The inductive RF source power applicator 8050 of FIG. 79 tends toexhibit a rapid increase in dissociation of fluorine-containing speciesin the plasma as plasma source power (and ion flux) is increased,causing undue etching of semiconductor films on the wafer during theimplantation process. Such etching is undesirable. A plasma immersionion implantation reactor that tends to avoid this problem is illustratedin FIG. 83A. The plasma immersion ion implantation reactor of FIG. 83Ahas a capacitive source power applicator constituting a conductive(metal) or semiconducting ceiling 8015′ electrically insulated from thegrounded side wall 8020 by an insulating ring 8017. Alternatively, theceiling may be metal, conductive, or semiconducting and be coating by aninsulating, conducting or semiconducting layer. The RF plasma sourcepower generator 8055 drives the ceiling 8015′ through the impedancematch circuit 8060 in the manner of a capacitive plate. Plasma isgenerated by oscillations in the plasma sheath produced by the RF powercapacitively coupled from the ceiling 8015′. In order to enhance suchplasma generation, the frequency of the plasma RF source power generator8055 is relatively high, for example within the very high frequency(VHF) range or 30 MHz and above. The wafer pedestal 8025 may serve as acounter electrode to the ceiling 8015′. The ceiling 8015′ may serve as acounter electrode to the RF bias voltage applied to the wafer pedestal8025. Alternatively, the chamber wall may serve as a counter electrodeto either or both wafer bias and ceiling bias voltages. In oneimplementation, the dopant-containing gas is fed through the ceiling8015′ through plural gas injection orifices 8048′.

[0253] The capacitively coupled plasma ion immersion implantationreactor of FIG. 83A enjoys the advantages of the inductively coupledreactor of FIG. 79 in that both types of reactors permit the independentadjustment of ion flux (by adjusting power level of the plasma sourcepower generator 8055) and of the ion energy or implant depth (byadjusting the power level of the plasma bias power generator 8065). Inaddition, when plasma source power or ion flux is increased, thecapacitively coupled plasma ion immersion reactor of FIG. 83A exhibits asmaller increase in dissociation of fluorine-containing species in thegas fed from the dopant gas supply 8045 and a smaller increase inreaction by-products which would otherwise lead to excessive etch ordeposition problems. The advantage is that ion flux may be increasedmore freely without causing an unacceptable level of etching ordeposition during ion implantation.

[0254] The higher frequency RF power of the plasma source powergenerator 8055 controls plasma density and therefore ion flux at thewafer surface, but does not greatly affect sheath voltage or ion energy.The lower frequency RF power of the bias power generator 8065 controlsthe sheath voltage and therefore the ion implantation energy and(junction) depth and does not contribute greatly to ion generation orion flux. The higher the frequency of the plasma source power generator,the less source power is wasted in heating ions in the plasma sheath, sothat more of the power is used to generate plasma ions throughoscillations of the plasma sheath or by heating electrons in the bulkplasma. The lower frequency of the RF bias power generator 8065 is lessthan 10 MHz while the higher frequency of the RF plasma source powergenerator 8055 is greater than 10 MHz. More preferably, the lowerfrequency is less than 5 MHz while the higher frequency is greater than15 MHz. Even better results are obtained with the lower frequency beingless than 3 MHz and the higher frequency exceeding 30 MHz or even 50MHz. In some cases the source power frequency may be as high as 160 MHzor over 200 MHz. The greater the separation in frequency between thehigher and lower frequencies of the source and bias power generators8055, 8065, respectively, the more the plasma ion implant flux and theplasma ion implant energy can be separately controlled by the twogenerators 8055, 8065.

[0255] In the variation illustrated in FIG. 83B, the RF plasma sourcepower generator 8055 is coupled to the wafer pedestal rather than beingcoupled to the ceiling 8015′. An advantage of this feature is that theceiling 8015′ is consumed (by plasma sputtering or etching) at a muchlower rate than in the reactor of FIG. 83A, resulting in less wear andless metallic contamination of the plasma. A disadvantage is thatisolation between the two RF generators 8055, 8065 from each other isinferior compared to the reactor of FIG. 83A, as they are both connectedto the same electrode, so that control of ion flux and ion energy is notas independent as in the reactor of FIG. 83A.

[0256] In either of the reactors of FIGS. 83A or 83B, the controller8075 can operate in the manner described above with reference to FIGS.82A through 82H, in which the respective RF power waveforms applied tothe ceiling 8015′ and the pedestal 8025 are in a push-pull mode (FIGS.82A and B), or an in-synchronism mode (FIGS. 82C and D), or a symmetricmode (FIGS. 802E and F) or a non-symmetric mode (FIGS. 82G and H).

[0257]FIGS. 83A and 83B show that the RF source power generator 8055 candrive the ceiling 8015′ (FIG. 83A) with the side wall 8020 and/or thewafer support pedestal 8025 connected to the RF return terminal of thegenerator 8055, or, in the alternative, the RF source power generator8055 can drive the wafer support pedestal 8025 with the ceiling 8015′and/or the sidewall 8020 connected to the RF return terminal of thegenerator 8055. Thus, the RF source power generator is connected acrossthe wafer support pedestal 8025 and the sidewall 8020 or the ceiling8015′ (or both). The polarity of the connections to the source powergenerator 8055 may be reversed, so that it drives the side wall 8020and/or ceiling 8015′ with the pedestal 8025 being connected to the RFreturn terminal of the generator 8055.

[0258] As set forth above, the plasma immersion ion implantationinductively coupled reactor of FIG. 79 has distinct advantages,including (a) the capability of a large ion flux/high plasma iondensity, (b) independently controlled ion energy, and (c) low minimumion energy (plasma potential). The plasma immersion ion implantationcapacitively coupled reactor of FIG. 83A has the additional advantage ofhaving more controllable dissociation of process gases and reactivebyproducts as ion flux is increased, than the inductively coupledreactor of FIG. 79. However, the capacitively coupled reactor of FIG.83A has a higher minimum ion energy/plasma potential than theinductively coupled reactor of FIG. 79. Thus, these two types ofreactors provide distinct advantages, but neither provides all of theadvantages.

[0259] A plasma immersion ion implantation reactor that provides all ofthe foregoing advantages, including low minimum ion energy and lowprocess gas dissociation, is illustrated in FIG. 84. In FIG. 84, theinductively or capacitively coupled plasma sources of FIG. 79 or 83A arereplaced by a torroidal plasma source of the type disclosed above inFIGS. 1-78. In the basic configuration of FIG. 84, the torroidal plasmasource includes a reentrant hollow conduit 8150 over the ceiling 8015,corresponding to the conduit 150 of FIG. 1. The conduit 8150 of FIG. 84has one open end 8150 a sealed around a first opening 8155 in theceiling 8015 and an opposite open end 8150 b sealed around a secondopening 8160 in the ceiling 8015. The two openings or ports 8155, 8160are located in the ceiling over opposite sides of the wafer supportpedestal 8025. While FIG. 84 illustrates the openings 8155, 8160 beingin the ceiling, the openings could instead be in the base or floor ofthe chamber, as in FIG. 12, or in the side wall of the chamber, as inFIG. 26, so that the conduit 8150 may pass over or under the chamber. RFplasma source power is coupled from the RF generator 8055 through theoptional impedance match circuit 8060 to the reentrant conduit by an RFplasma source power applicator 8110. Various types of source powerapplicators for a reentrant hollow conduit are disclosed in FIGS. 1-78,any one of which may be employed in the plasma immersion ionimplantation reactor of FIG. 84. In the implementation illustrated inFIG. 84, the RF plasma source power applicator 8110 is similar to thatillustrated in FIG. 13, in which a magnetically permeable core 8115having a torus shape surrounds an annular portion of the conduit 8150.The RF generator 8055 is coupled through the optional impedance matchcircuit to a conductive winding 8120 around the magnetic core 8115. Anoptional tuning capacitor 8122 may be connected across the winding 8120.The RF generator 8055 may be frequency-tuned to maintain an impedancematch, so that the impedance match circuit 8060 may not be necessary.

[0260] The reactor chamber includes the process region 8140 between thewafer support pedestal 8025 and the ceiling 8015. The gas supply 8045furnishes dopant gases into the reactor chamber 8140 through gasinjection orifices 8048 in the ceiling 8015. Plasma circulates(oscillates) through the reentrant conduit 8150 and across the processregion 8140 in response to the RF source power coupled by the sourcepower applicator 8110. As in the reactor of FIG. 13, the reentrantconduit 8150 is formed of a conductive material and has a narrow gap orannular break 8152 filled with an insulator 8154. The dopant gasesfurnished by the gas supply 8045 contain a species that is either adonor (N-type) or acceptor (P-type) impurity when substituted into thesemiconductor crystal structure of the wafer 8030. For example, if thewafer is a silicon crystal, then an N-type dopant impurity may bearsenic or phosphorous, for example, while a P-type dopant impurity maybe boron, for example. The dopant gas furnished by the gas supply 8045is a chemical combination of the dopant impurity with an at-leastpartially volatile species, such as fluorine for example. For example,if a P-type conductivity region is to be formed by ion implantation,then the dopant gas may be a combination of boron and fluorine, such asBF₃, for example. Or, for example, the dopant gas be a hydride, such asB₂H₆. Phosphorous doping may be accomplished using a fluoride such asPF₃ or PF₅ or a hydride such as PH₃. Arsenic doping may be accomplishedusing a fluoride such as AsF₅ or a hydride such as AsH₃.

[0261] The RF bias power generator provides an RF bias voltage, with theRF bias frequency selected as described above with reference to FIG.81D. Good results are attained by restricting the RF bias powerfrequency to a low frequency range between 10 kHz and 10 MHz. Betterresults are obtained by limiting the RF bias power frequency to anarrower range of 50 kHz to 5 MHz. The best results are obtained in theeven narrower bias power frequency range of 100 kHz to 3 MHz. We havefound optimum results at about 2 MHz plus or minus 5%.

[0262] In the reactor of FIG. 84, both the RF source power generator8055 and the RF bias power generator 8065 may apply continuous RF powerto the inductive power applicator 8110 and the wafer pedestal 8025respectively. However, either or both of the generators 8055, 8065 maybe operated in burst modes controlled by a controller 8075. The RFsignals produced by each of the generators 8055, 8065 may be pulsemodulated to produce continuous wave (CW) RF power in bursts lasting,for example, one millisecond with a repetition rate on the order of 0.5kHz, for example. Either one or both of the RF power generators 8055,8065 may be operated in this manner. If both are operated in such aburst mode simultaneously, then they may be operated in a push-pullmode, or in an in-synchronism mode, or in a symmetrical mode or in anon-symmetrical mode, as will now be described for the reactor of FIG.84.

[0263] A push-pull mode is illustrated in the contemporaneous timedomain waveforms of FIGS. 82A and 82B, illustrating the RF powerwaveforms of the respective RF generators 8055 and 8065, in which thebursts of RF energy from the two generators 8055, 8065 occur duringalternate time windows. FIGS. 82A and 82B illustrate the RF powerwaveforms of the generators 8055 and 8065, respectively, or vice versa.

[0264] An in-synchronism mode is illustrated in the contemporaneous timedomain waveforms of FIGS. 82C and 82D, in which the bursts of RF energyfrom the two generators 8055, 8065 are simultaneous. They may not benecessarily in phase, however, particularly where the two generators8055, 8065 produce different RF frequencies. For example, the RF plasmasource power generator 8055 may have a frequency of about 13 MHz whilethe RF plasma bias power generator 8065 may have a frequency of about 2MHz. FIGS. 82C and 82D illustrate the RF power waveforms of thegenerators 8055 and 8065, respectively, or vice versa.

[0265] In the foregoing examples, the pulse widths and pulse repetitionrates of the two RF generators 8055, 8065 may be at least nearly thesame. However, if they are different, then the temporal relationshipbetween the bursts of the two generators 8055, 8065 must be selected. Inthe example of the contemporaneous time domain waveforms of FIGS. 82Eand 82F, one of the generators 8055, 8065 produces shorter RF burstsillustrated in FIG. 82F while the other produces longer RF burstsillustrated in FIG. 82E. In this example, the bursts of the twogenerators 8055, 8065 are symmetrically arranged, with the shorterbursts of FIG. 82F centered with respect to the corresponding longerbursts of FIG. 82E. FIGS. 82E and 82F illustrate the RF power waveformsof the generators 8055 and 8065, respectively, or vice versa.

[0266] In another example, illustrated in the contemporaneous timedomain waveforms of FIGS. 82G and 82H, the shorter bursts (FIG. 82H) arenot centered relative to the corresponding longer bursts (FIG. 82G), sothat they are asymmetrically arranged. Specifically, in this example theshorter RF bursts of FIG. 82H coincide with the later portions ofcorresponding ones of the long bursts of FIG. 82G. Alternatively, asindicated in dashed line in FIG. 82H, the short RF bursts of FIG. 82Hmay instead coincide with the earlier portions of corresponding ones ofthe long RF bursts of FIG. 82G. FIGS. 82G and 82H illustrate the RFpower waveforms of the generators 8055 and 8065, respectively, or viceversa.

[0267] The torroidal plasma immersion ion implantation reactor of FIG.84 can be operated with a pulsed D.C. bias voltage instead of an RF biasvoltage. In this case, the bias power generator 8065 would be D.C.source rather than an RF source. Thus, in the different operationalmodes of FIGS. 82A through 82H discussed above, the pulsed RF biasvoltage may be replaced by a pulsed D.C. bias voltage of the same pulsewidth, with only the source power generator 8055 producing an RF powerburst.

[0268]FIG. 85 illustrates a modification of the plasma immersion ionimplantation reactor of FIG. 84 having a second reentrant conduit 8151crossing the first reentrant conduit 8150, in a manner similar to thereactor of FIG. 44. Plasma power is coupled to the second conduit 8151from a second RF plasma source power generator 8056 through a secondoptional match circuit 8061 to a second source power applicator 8111that includes a second magnetically permeable core 8116 and a secondcore winding 8121 driven by the second RF source power generator 8056.Process gas from the gas supply 8045 may be introduced into the chamberby a gas distribution plate or showerhead incorporated in the ceiling8015 (as in the gas distribution plate 210 of FIG. 44). However, theplasma immersion ion implantation reactor of FIG. 85 is greatlysimplified by using a small number of process gas injectors 8048 in theceiling 8015 or in the side wall 8020 or elsewhere, such as in the baseof the chamber (not shown) coupled to the dopant gas supply, rather thana showerhead. Moreover, the gap between the ceiling 8015 and the waferpedestal 8025 may be relatively large (e.g., two to six inches) and agas distribution plate eliminated in favor of discrete gas injectors ordiffuser 8048 in the ceiling 8015 or gas injectors or diffusers 8049 inthe side wall 8020 because there is no need to generate plasma close tothe wafer surface. The gas injectors or diffusers 8049 may be joined ina ring 8049 on the side wall 8020. Generally, the higher the maximumimplant depth and ion energy requirement, the greater the gap betweenceiling and wafer that is required. For example, for a peak-to-peak RFbias voltage of 10 kV, a gap of 4 inches is preferable over a 2 inch gapfor best plasma uniformity across a wide range of gas species and plasmaelectron densities. The term diffuser is employed in the conventionalsense as referring to a type of gas distribution device having a wideangle of gas flow distribution emanating from the device.

[0269]FIG. 86 is a plan view of the interior surface of the ceiling8015, showing one arrangement of the gas injection orifices 8048, inwhich there is one central orifice 8048-1 in the center of the ceiling8015 and four radially outer orifices 8048-2 through 8048-5 uniformlyspaced at an outer radius. FIG. 87 illustrates how the dopant gas supply8045 may be implemented as a gas distribution panel. The gasdistribution panel or supply 8045 of FIG. 87 has separate gas reservoirs8210-1 through 8210-11 containing different dopant-containing gasesincluding fluorides of boron, hydrides of boron, fluorides ofphosphorous and hydrides of phosphorous. In addition, there are gasreservoirs for other gases used in co-implantation (hydrogen andhelium), material enhancement (nitrogen), surface passivation orco-implantation (fluorides of silicon or germanium or carbon). Inaddition, the center orifice 8048-1 may be coupled to a reservoir oxygengas, for use in photoresist removal and/or chamber cleaning. A controlpanel 8220 includes valves 8222 controlling gas flow from the respectivereservoirs 8210 to the gas injection orifices. Preferably, the gases aremixed at or near the orifices, although a gas manifold 8230 may beprovided to distribute the selected gases among the outer gas injectionorifices 8048-2 through 8048-5. Alternatively, process gas may beinjected at one or more locations in the sidewall 8020, using thenozzles 8049 of FIG. 85 or diffusers. FIG. 85 shows gas injectors 8049located around the chamber sidewalls 8020 which inject gas radiallyinward. Gas may be injected parallel to the ceiling and/or wafer, or maybe injected with some component toward ceiling and/or wafer. For someapplications, it is advantageous to utilize multiple separate gasplenums, each with its own nozzle array. This can permit the use ofchemistries which should not be combined except under vacuum, or maypermit having several gas zones for neutral uniformity tuning. For thispurpose, referring again to FIG. 85, a first ring 8049 a joining a firstset of side wall injectors 8049 c serves as a first plenum, while asecond ring 8049 b joining a second separate set of side wall injectors8049 d serves as a second plenum. The two rings or plenums 8049 a, 8049b are supplied by separate respective sets of valves 8222 of the gaspanel of FIG. 87

[0270]FIG. 88 illustrates a modification of the plasma immersion ionimplantation reactor of FIG. 85 in which a central electromagnetassembly 8430 is mounted over the center of the ceiling 8015. Like theelectromagnet assembly 4430 of FIG. 44, the electromagnet assembly 8430of FIG. 88 controls plasma ion density uniformity and includes a narrowelongate cylindrical pole piece 8440 formed of a magnetizable materialsuch as iron or steel and a coil 8450 of insulated conductive wirewrapped around the pole piece 8440. A magnetic current controller 8442supplies an electrical current to the coil 8450. The controller 8442controls the current through the coil 8450 so as to optimize uniformityof plasma ion density (ion flux) across the wafer surface.

[0271]FIGS. 89A and 89B are side and top views, respectively,illustrating a further modification incorporating a radially outerelectromagnet assembly 8460. The outer electromagnet assembly 8460 is inthe shape of a torus and overlies an annular outer region of the ceiling8015 near the circumferential edge of the ceiling 8015 and adjacent theports pairs 150, 160 of the conduits 8150, 8151. Referring to thecross-sectional view of FIG. 90A, the outer electromagnet assembly 8460includes a coil 8462 consisting of plural windings of a single conductorconnected to the current controller 8442. In order to concentrate themagnetic field of the outer electromagnet assembly 8460 within theprocess region 8140, an overlying magnetic cover 8464 surrounding thesides and top of the coil 8462 but not the bottom of the coil 8462. Themagnetic cover 8464 permits the magnetic field of the coil 8462 toextend downwardly below the ceiling into the process region 8140.Uniformity of the ion density and radial plasma flux distribution at thewafer surface is optimized by independently adjusting the currents inthe inner and outer electromagnet assemblies 8430, 8460.

[0272] In order to avoid forming regions of very high plasma ionconcentration near the ports 150, 160 of the two conduits 8150, 8151,individual plates 8466 of magnetically permeable material (e.g., iron orsteel) are placed under the outer electromagnet assembly 8460 adjacentrespective ones of the ports 150, 160. The circumferential extent ofeach plate 8466 is approximately equal to the width of each individualport 150, 160. FIGS. 90A, 90B and 90C are cross-sectional views takenalong lines 90-90 of FIG. 89B. The distance between the plate 8466 andthe bottom edge of the magnetic cover 8464 may be adjusted to controlthe amount of magnetic field coupled into portion of the process regionnear each individual one of the ports 150, 160. In FIG. 90A, the plate8466 is in contact with the bottom edges of the cover 8464, so that themagnetic field near the corresponding port (150, 160) is almostcompletely confined within the enclosure defined by the cover 8464 andthe plate 8466. In FIG. 90B, the plate 8466 is slightly displaced fromthe bottom edge of the cover 8464, creating a small gap therebetweenthat allows a small magnetic field to enter the process region 8140 nearthe corresponding port (150, 160). In FIG. 90C, there is a large gapbetween the plate 8466 and the cover 8464, permitting a larger magneticfield to exist in the process region near the corresponding port (150,160).

[0273]FIG. 91 illustrates how the RF plasma bias power generator 8065may be coupled to the wafer support pedestal 8025. An inductor 8510 anda variable capacitor 8520 are connected in parallel between one side ofa series capacitor 8530 and ground, the other side of the seriescapacitor 8530 being connected to the wafer support pedestal 8025. Theoutput of the bias power generator 8065 is connected to a tap 8560 ofthe inductor 8510. The position of the tap 8560 and the capacitance ofthe variable capacitor 8520 are selected to provide an impedance matchbetween the bias power generator 8065 and the plasma load at the waferpedestal 8065. The variable capacitor 8520 may be controlled by a systemcontroller 8525 to optimize matching. In this case, the circuitincluding the parallel inductor and capacitor 8510, 8520 serves as animpedance match circuit. In order to follow variations in the plasmaload impedance during processing, frequency tuning of the bias powergenerator 8065 may be employed, although this may not be necessary. Theposition of the tap 8560 may be selectable either manually or by thesystem controller 8525 to optimize matching. Alternatively, a capacitor(not shown) may be connected between the tap position and ground orbetween RF bias generator and tap point as an alternative matchingcircuit topology. This optional capacitor may be controlled by thesystem controller 8525 to optimize matching.

[0274] One problem in selecting the bias voltage level is that large ionenergy can be reached only with a high bias voltage level, whichtypically requires high power. High power contributes to the plasma flux(ion density or dose rate), and can cause too high a dose rate, makingit difficult to control the conductivity of the implanted region. Oneway of controlling the dose rate at such a high power is to pulse the RFbias power. However, controlling the pulse rate and pulse width ofrepetitive pulses so as to achieve the required dose rate andconductivity is difficult. Part of the problem is that ion implantationat the desired junction depth is achieved only after the bias voltagehas risen sufficiently (at the beginning of a pulse or RF burst) toreach a threshold voltage corresponding to the desired junction depthand ion energy. The solution to this problem is to avoid repetitivepulsing of the bias power, and instead use a single pulse of sufficientduration to complete ion implantation at the desired junction depth andconductivity in the implanted region. This is illustrated in the timedomain waveform of FIG. 92. A timer can be employed to guarantee thatthe RF burst or pulse lasts the required duration (Ttimer). However, thetimer must not begin until the sheath voltage has reached the thresholdvoltage (Vthreshold) at which ion implantation occurs at the requireddepth. Thus, FIG. 92 shows that the sheath voltage grows at thebeginning of bias power RF burst (Ton) until it reaches Vthreshold afterseveral cycles. At that point, the timer begins, and ends the RF burstat the expiration of Ttimer, i.e., at Toff. The problem, therefore, ishow to ascertain the time at which the sheath voltage reachesVthreshold, i.e., when to begin Ttimer.

[0275] Another problem is how to ascertain the requisite power level ofthe bias power generator 8065 at which Vthreshold is produced across thesheath.

[0276]FIG. 93 illustrates a control circuit for determining the biasgenerator power level that produces the desired sheath voltage and fordetermining when the target sheath voltage has been reached forbeginning the RF burst timer. In the following description, the targetbias voltage corresponding to a desired junction depth, has already beendetermined. In addition, the threshold voltage for implantation has alsobeen determined, and the threshold voltage may be synonymous with thetarget bias voltage. Finally, the duration time for applying RF biaspower at the target bias voltage has already been determined. The RFbias power generator 8065 is controlled by a timer 8670 that beginscounting sometime after the beginning of an RF burst and times out aftera predetermined duration. A threshold comparator 8672 compares thepeak-to-peak voltage as detected at the wafer pedestal 8025 by a peakdetector 8674 with the desired threshold voltage 8676. The timer 8670 isenabled only when it receives an affirmative signal from an opticaldetector 8678 indicating that plasma is ignited within the reactorchamber. If the optical detector 8678 sends an affirmative signal, thenthe timer 8670 begins counting as soon as the comparator 8672 determinesthat the peak-to-peak bias voltage has reached the desired threshold.When the timer 8670 times out (after the predetermined duration), itturns off the output of the bias power generator, thus terminating thecurrent burst of RF bias power. The timer 8670 and the thresholdcomparator 8672 constitute a timer control loop 8680.

[0277] The power level of the bias power generator 8065 is controlled bya voltage control loop 8682. A process controller 8684 (or the processdesigner) determines the desired or “target” bias peak-to-peak voltage.This may be synonymous with the threshold voltage of 8676. A subtractor8686 computes an error value as the difference between the actual peakbias voltage measured by the detector 8674 and the target bias voltage.A proportional integral conditioner 8688 multiplies this error value bya constant of proportionality, k, and integrates the error value withprior samples. The result is an estimated correction to the power levelof the bias power generator 8065 that will bring the measured biasvoltage closer to the target bias voltage. This estimate is superimposedon the current power level, and the result is an estimated power levelcommand that is applied to the power set input of the bias powergenerator 8065. This estimate is only valid while plasma is ignited(i.e., during an RF burst). For times between RF bursts, the bias powerlevel is controlled in accordance with a look-up table 8690 thatcorrelates target peak-to-peak bias voltages with estimated bias powerlevels. The look-up table receives the target bias voltage from theprocess controller 8684 and in response outputs an estimated bias powerlevel. A pair of switches 8694, 8696 are enabled in complementaryfashion by the output of the plasma ignition optical detector 8678.Thus, the switch 8694 receives the output of the sensor 8678 while theswitch 8696 receives the inverted output of the sensor 8678. Thus,during an RF burst, when plasma is ignited in the chamber, the output ofthe proportional integral conditioner 8688 is applied to the power setinput of the bias generator 8065 via the switch 8694. Between RF bursts,or when no plasma is ignited in the chamber, the output of the look-uptable 8690 is applied via the switch 8696 to the power set input of thebias power generator 8065. The output of the look up table 8690 may beconsidered as a gross estimate that serves to initialize the RF biaspower level at the beginning of each RF burst, while the output of theintegral proportional conditioner is a more accurate estimate based uponactual measurement that serves to correct the bias power level duringthe RF burst.

[0278] One problem in the plasma immersion ion implantation reactor ofFIG. 89A is that most ion implantation processes must be carried outwith precise fine control over chamber pressure. This requires arelatively gradual change in chamber pressure over a given rotation ofthe control valve 8037 from its closed position. On the other hand, someprocesses, including chamber cleaning, require a very high gas flow rate(e.g., of cleaning gases) and a concomitantly high evacuation rate bythe pump 8035. This requires that the vacuum control valve 8037 have alarge area. The problem is that with such a large area, a vacuum controlvalve does not provide the gradual change in pressure for a givenrotation from its closed position that is necessary for fine control ofchamber pressure during ion implantation. In fact, with a large areaopening and flap, the change in chamber pressure is very rapid as theflap is rotated from its closed position, so that fine control ofpressure within a very low pressure range, where the flap must be nearlyclosed, is very difficult. This problem is solved with the vacuumcontrol valve of FIGS. 94, 95 and 96. The valve includes a flat housing9410 having a circular opening 9412 through it. A rotatable flap 9420having a disk shape is supported within the circular opening 9412 by ahinge 9422 attached to the housing 9410. In its closed position, theflap 9420 is co-planar with the flat housing 9410. In order to preventleakage of plasma through the valve, the gap G between the rotatableflap 9420 and the housing 9410 is narrow while the thickness T of theflap 9420 and housing 9410 is large, much greater than the gap G. Forexample, the ratio of the thickness T to the gap G is about 10:1. Thisfeature provides the advantage of frictionless operation. In order toprovide gradual control of chamber pressure at a very low pressure range(i.e., when the flap 9420 is near its closed position), conically-shapedopenings 9430 are provided in the interior surface 9440 of the housing9410 defining the edge of the opening 9412. Some of the openings 9430have different axial locations (along the axis of the opening 9412) thanothers of the openings 9430. In its closed position, the flap 9420permits virtually zero gas leakage, because the openings 9430 are notexposed. As the flap 9420 begins to rotate from its closed position(i.e., in which the flap 9420 is co-planar with the housing 9410), smallportions of at least some of the openings 9430 begin to be exposed, andtherefore allow a small amount of gas flow through the valve. As theflap 9420 continues to rotate, it exposes larger portions of theopenings 9430. Moreover, it begins to expose others of the openings 9430not exposed during the earlier phase of its rotation due to thedifferent axial locations of different sets of the openings 9430, sothat the gas flows through more of the openings 9430 in proportion tothe rotation of the flap 9420. Thus, rotation of the flap 9430 from itsfully closed (co-planar) position causes a continuous but relativelygradual increase in gas flow through the openings 9430 until the bottomedge 9420 a of the flap 9420 reaches the top surface 9410 a of thehousing 9410. At this point, all of the openings 9430 are completelyexposed so that gas flow through the openings 9430 is maximum and cannotincrease further. Thus, a continuous gradual increase in gas flow isachieved (and therefore one that is readily controlled with a great dealof accuracy) as the flap 9420 rotates from its fully closed position tothe point at which the flap bottom edge 9420 a is aligned with thehousing top surface 9410 a. Within this range of flap rotationalposition, fine gradual adjustment of a small total chamber pressure isprovided. Further rotation of the flap 9420 creates an annular gapbetween the periphery of the flap 9420 and the periphery of the largecircular opening 9412, through which gas flow increases as the flap 9420continues to rotate.

[0279] The plural openings 9430 in the opening interior surface 9440 aresemi-circular openings that are tapered so as to increase in diametertoward the top housing surface 9410 a. The tapered semi-circularopenings 9430 thus define semi-conical shapes. However, other suitableshapes may be employed, such as semi-cylindrical, for example. However,one advantage of the semi-conical shape is that the rate of increase ofgas flow with rotational flap position may be enhanced as the rotationprogresses so that the rate continues to increase in a fairly smoothmanner after the transition point at which the flap bottom edge 9420 apasses the housing top surface 9410 a.

[0280] Depending upon the desired junction depth, the RF bias voltageapplied to the wafer support pedestal 8025 may be relatively small(e.g., 500 volts) for a shallow junction or relatively large (e.g.,5,000 volts) for a deep junction. Some applications may require an RFbias voltage of over 10,000 volts. Such large voltages can cause arcingwithin the wafer support pedestal 8025. Such arcing distorts processconditions in the reactor. In order to enable the wafer support pedestal8025 to withstand bias voltages as high a 10,000 volts, for example,without arcing, voids within the wafer support pedestal 8025 are filledwith a dielectric filler material having a high breakdown voltage, suchas Rexolite®, a product manufactured by C-Lec Plastics, Inc. Asillustrated in FIG. 97, the wafer support pedestal 8025 consists of agrounded aluminum base plate 9710, an aluminum electrostatic chuck plate9720 and a cylindrical side wall 9730. Dielectric filler material 9735fills voids between the side wall 9730 and the electrostatic chuck plate9720. Dielectric filler material 9737 fills voids between theelectrostatic chuck plate 9720 and the base plate 9710. A coaxial RFconductor 9739 carrying the RF bias power from the RF generator 8065(not shown in FIG. 97) is terminated in a narrow cylindrical conductivecenter plug 9740 that fits tightly within a matching conductivereceptacle 9742 of the electrostatic chuck plate 9720. A wafer lift pin9744 (one of three) extends through the pedestal 8025. The lift pin 9744is tightly held within the electrostatic chuck plate 9720 by asurrounding blanket 9746 of the dielectric filler material. A void 9748that accommodates a guide 9750 of the lift pin 9744 is located entirelywithin the base plate 9710 so as to minimize the risk of arcing withinthe void 9748. Referring to FIG. 98, bolt 9754 (one of several) holdingthe base plate 9710 and the electrostatic chuck plate 9720 together iscompletely encapsulated to eliminate any voids around the bolt 9754,with dielectric layers 9756, 9758 surrounding exposed portions of thebolt 9754. The foregoing features have been found to enable the wafersupport pedestal to withstand an RF bias voltage of over 10,000 voltswithout experiencing arcing.

[0281]FIG. 99 illustrates an ion implantation system including a plasmaimmersion ion implantation reactor 9910 of the type illustrated in FIGS.79, 83A, 83B, 84, 85, 88, 89A or 93. An independent source 9920 ofchamber-cleaning radicals or gases (such as fluorine-containing gases orfluorine-containing radicals like NF₃ and/or other cleaning gases suchas hydrogen-containing gases (e.g., H₂ or compounds of hydrogen) toproduce hydrogen-containing radicals or oxygen-containing gases (e.g.,O₂) is coupled to the implant reactor 9910 for use during chambercleaning operations. A post-implant anneal chamber 9930 and an ion beamimplanter 9940 are also included in the system of FIG. 99. In addition,an optical metrology chamber 9950 may also be included. Furthermore, aphotoresist pyrolization chamber 9952 may be included in the system forremoval of the photoresist mask subsequently after implant and prior toanneal. Alternatively, this may be accomplished within the plasmaimmersion implantation reactor 9910 using the RF plasma source power andoptional bias power with oxygen gas, and/or by using the independentself-cleaning source with oxygen gas.

[0282] The system of FIG. 99 may also include a wet clean chamber 9956for carrying out wafer cleaning. The wet clean chamber 9956 may employsuch well known wet cleaning species as HF, for example. The wet cleanchamber 9956 may be employed for pre-implantation or post-implantationcleaning of the wafer. The pre-implantation cleaning use of the wetclean chamber 9956 may be for removing a thin native oxide that canaccumulate on the wafer between processing operations. Thepost-implantation cleaning use of the wet clean chamber 9956 may be forremoving photoresist from the wafer in lieu of the photoresist stripchamber 9952. The system of FIG. 99 may further include a second,(third, fourth or more) plasma immersion ion implantation reactor 9958of the type illustrated in FIGS. 79, 83A, 83B, 84, 85, 88, 89A or 93. Inone example, the first PIII reactor 9910 may be configured to ionimplant a first species while the second PIII reactor 9958 may beconfigured to implant a second species, so that a single PIII reactorneed not be re-configured to implant the two species in each wafer.Furthermore, the first and second species may be dopant impurities foropposite semiconductor conductivity types (e.g., boron and phosphorus),in which case the second PIII reactor 9958 may be employed in lieu ofthe beam implantation tool 9940. Or, two N-type dopants (phosphorous andarsenic) may be implanted in addition to a P-type dopant (boron), inwhich case boron implantation is carried out by the first PIII reactor9910, arsenic implantation is carried out in the ion beam tool 9940 andphosphorus implantation is carried out in the second PIII reactor 9958,for example. In another example, the 2 (or more) PIII reactors may beconfigured to implant the same species so as to increase the throughputof the system.

[0283] A between the plasma ion implant reactor 9910, the anneal chamber9930, the ion beam implanter 9940, the photoresist pyrolization chamber9952, the optical metrology chamber 9950, the wet clean chamber 9956 andthe second PIII reactor 9958. If the entire system of FIG. 99 isprovided on a single tool or frame, the handler 9945 is a part of thattool and is supported on the same frame. However, if some of thecomponents of the system of FIG. 99 are on separate tools located inseparate places in a factory, then the handler 9945 is comprised ofindividual handlers within each tool or frame and a factory interfacethat transports wafers between tools within the factory, in thewell-known manner. Thus, some or all of the components of the wafertransfer robotic handler 9945 transfers wafers system of FIG. 99 may beprovided on a single tool with its own wafer handler 9945.Alternatively, some or all of the components of the system of FIG. 99may be provided on respective tools, in which case the wafer handler9945 includes the factory interface.

[0284] The process controller 8075 can receive measurements of apreviously implanted wafer from the optical metrology chamber 9950, andadjust the implant process in the plasma implant reactor 9910 for laterwafers. The process controller 8075 can use established data miningtechniques for process correction and control. The inclusion of the ionbeam implanter 9940 permits the system to perform all of the ionimplantation steps required in semiconductor fabrication, includingimplantation of light elements (such as boron or phosphorous) by theplasma ion implant reactor 9910 and implantation of heavier elements(such as arsenic) by the ion beam implanter 9940. The system of FIG. 99may be simplified. For example, a first version consists of only thechamber cleaning radical source 9920, the PIII reactor 9910 and theprocess controller 8075. A second version includes the foregoingelements of the first version and, in addition, the optical metrologytool 9950. A third version includes the foregoing elements of the secondversion and, in addition, the ion beam implanter 9940 and/or the secondPIII reactor 9958. A fourth version includes the foregoing elements ofthe third version and, in addition, the anneal chamber 9930.

Ion Implantation Performance of the Torroidal Source

[0285] The plasma immersion ion implantation (PIII) reactor of FIG. 85realizes many advantages not found heretofore in a single reactor.Specifically, the PIII reactor of FIG. 85 has low minimum ion implantenergy (because it has a low plasma potential), low contamination(because the recirculating plasma generally does not need to interactwith chamber surfaces to provide a ground return), very good controlover unwanted etching (because it exhibits low fluorine dissociation),and excellent control over ion implant flux (because it exhibits anearly linear response of plasma electron density to source power).

[0286] The advantage of excellent control over ion implant flux isillustrated in the graph of FIG. 100, in which electron density isplotted as a function of source power level for the torroidal sourcePIII reactor of FIG. 85 and for an inductively coupled PIII reactor ofthe type illustrated in FIG. 79. Electron density is an indicator ofplasma ion density and therefore of the ion implant flux or implant doseto the wafer. The inductively coupled source of the PIII reactor of FIG.79 tends to have a highly non-linear response of electron density toapplied source power, exhibiting a sudden increase in electron densityat a threshold power level, PICP, below which the slope (response) isnegligible and above which the slope (response) is so steep thatelectron density (and therefore ion implant flux or dose) is nearlyimpossible to control to any fine degree. In contrast the torroidalsource PIII reactor of FIG. 85 has a generally linear and gradualresponse of electron density to source power level above a thresholdpower level PTH, so that ion implant flux (dose) is readily controlledto within a very fine accuracy even at very high source power level. Itshould be noted here that the plasma source power level of the torroidalsource PIII reactor of FIG. 85 is a function of the two different sourcepower generators 8055, 8056 coupled to the respective reentrant conduits8150, 8151. The source power frequency may be about 13.56 MHz, althoughthe frequency of each of the two source power generators 8055, 8056 areoffset from this frequency (13.56 MHz) by +100 kHz and −100 kHz,respectively, so that the two torroidal plasma current paths establishedby the sources 8110 and 8111 are decoupled from one another by beingde-tuned from one another by about 200 kHz. However, their power levelsmay be generally about the same. Operating frequencies are not limitedto the regime described here, and another RF frequency and frequencyoffset may be selected for the pair of RF source power generators 8055,5056.

[0287] The advantage of low fluorine dissociation of the PIII reactor ofFIG. 85 is important in preventing unwanted etching that can occur whena fluorine-containing dopant gas, such as BF3, is employed. The problemis that if the BF3 plasma by-products are dissociated into the simplerfluorine compounds, including free fluorine, the etch rate increasesuncontrollably. This problem is solved in the PIII reactor of FIG. 85 bylimiting the fluorine dissociation even at high power levels and highplasma density. This advantage is illustrated in the graph of FIG. 101,in which free fluorine density (an indicator of fluorine dissociation)is plotted as a function of source power for the PIII reactor of FIG. 85and for the inductively coupled reactor of FIG. 79 for the sake ofcomparison. The inductively coupled reactor of FIG. 79 exhibits anextremely sudden increase in free fluorine density above a particularsource power level, PDIS, above which the dissociation increases at avery high rate of change, and is therefore difficult to control. Incontrast, the PIII reactor of FIG. 85 exhibits generally linear andnearly negligible (very gradual) increase in free fluorine density abovea threshold source power PTH. As a result, there is very little unwantedetching during ion implantation with fluorine-containing dopant gases inthe torroidal source PIII reactor of FIG. 85. The etching is furtherminimized if the temperature of the wafer is held to a low temperature,such as below 100 degrees C., or more preferably below 60 degrees C., ormost preferably below 20 degrees C. For this purpose, the wafer pedestal8025 may be an electrostatic chuck that holds and releases the waferelectrostatically with thermal control cooling apparatus 8025 a and/orheating apparatus 8025 b that control the temperature of a semiconductorwafer or workpiece held on the top surface of the wafer support pedestal8025. Some small residual etching (such as may be realized with thetorroidal source PIII reactor of FIG. 85) is acceptable and may actuallyprevent the deposition of unwanted films on the wafer during ionimplantation. During ion implantation, some plasma by-products maydeposit as films on the wafer surface during ion implantation. This isparticularly true in cases where the implantation process is carried outat a very low ion energy (low bias voltage) and particularly with adopant gas consisting of a hydride of the dopant species (e.g., ahydride of boron or a hydride of phosphorous). In order to furtherreduce unwanted depositions that normally occur with hydride dopants(e.g., B₂H₆, PH₃), one aspect of the process is to add hydrogen and/orhelium to the dopant gas to eliminate the deposition on the surface ofthe wafer. However, the requisite etch rate to compete with such anunwanted deposition is very low, such as that exhibited by the torroidalsource PIII reactor of FIG. 85.

[0288] The advantage of a low minimum ion implant energy increases therange of junction depths of which the PIII reactor of FIG. 85 is capable(by reducing the lower limit of that range). This advantage isillustrated in the graph of FIG. 102, in which plasma potential isplotted as a function of plasma source power for the torroidal sourcePIII reactor of FIG. 85 and for the capacitively coupled PIII reactor ofFIG. 83A, for the sake of comparison. The plasma potential is thepotential on ions at the wafer surface due to the plasma electric fieldin the absence of any bias voltage on the wafer, and therefore is anindicator of the minimum energy at which ions can be implanted. FIG. 102shows that the plasma potential increases indefinitely as the sourcepower is increased in the capacitively coupled PIII reactor of FIG. 83A,so that in this reactor the minimum implant energy is greatly increased(the implant energy/depth range is reduced) at high plasma density orion implant flux levels. In contrast, above a threshold power PTH, thetorroidal source PIII reactor of FIG. 85 exhibits a very gradual (nearlyimperceptible) increase in plasma potential as source power isincreased, so that the plasma potential is very low even at high plasmasource power or ion density (high ion implant flux). Therefore, therange of plasma ion energy (ion implant depth) is much larger in thePIII reactor of FIG. 85 because the minimum energy remains very low evenat high ion flux levels.

[0289] The plasma potential in the capacitively coupled PIII reactor ofFIG. 83A can be reduced by increasing the source power frequency.However, this becomes more difficult as the junction depth andcorresponding ion energy is reduced. For example, to reach a plasmapotential that is less than 500 eV (for a 0.5 kV Boron implant energy),the source power frequency would need to be increased well into the VHFrange and possibly above the VHF range. In contrast, the source powerfrequency of the torroidal source PIII reactor of FIG. 85 can be in theHF range (e.g., 13 MHz) while providing a low plasma potential.

[0290] A further advantage of the torroidal source PIII reactor of FIG.85 over the capacitively coupled source PIII reactor of FIG. 83A is thatthe torroidal source PIII reactor has a thinner plasma sheath in whichproportionately fewer inelastic collisions of ions occur that tend toskew the ion implant energy distribution. This thinner sheath may benearly collisionless. In contrast, the capacitively coupled source PIIIreactor of FIG. 83A generates plasma ions in the sheath by an HF or VHFRF source that tends to produce a much thicker sheath. The thickersheath produces far more collisions that significantly skew ion energydistribution. The result is that the ion implanted junction profile isfar less abrupt. This problem is more acute at lower ion energies(shallower implanted junctions) where the skew in energy produced by thecollisions in the thicker sheath represent a far greater fraction of thetotal ion energy. The torroidal source PIII reactor of FIG. 85 thereforehas more precise control over ion implant energy and is capable ofproducing implanted junctions with greater abruptness, particularly forthe more shallow junctions that are needed for the more advanced(smaller feature size) technologies.

[0291] A related advantage of the torroidal source PIII reactor of FIG.85 is that it can be operated at much lower chamber pressures than thecapacitively coupled PIII reactor of FIG. 83A. The capacitively coupledPIII reactor of FIG. 83A requires a thicker sheath to generate plasmaions in the sheath, which in turn requires higher chamber pressures(e.g., 10-100 mT). The torroidal source PIII reactor of FIG. 85 does notneed to generate plasma near the sheath with bias power and for manyapplications therefore is best operated with a thinner (nearlycollisionless) sheath, so that chamber pressures can be very low (e.g.,1-3 mT). This has the advantage of a wider ion implantation processwindow in the torroidal source PIII reactor. However, as will bediscussed with reference to doping of a three dimensional structure suchas a polysilicon gate having both a top surface and vertical side walls,velocity scattering of dopant ions in the sheath enables ions to implantnot only the top surface of the polysilicon gate but also implant itsside walls. Such a process may be referred to as conformal ionimplanting. Conformal ion implanting has the advantage of doping thegate more isotropically and reducing carrier depletion at thegate-to-thin oxide interface, as will be discussed below. Therefore,some sheath thickness is desirable in order to scatter a fraction of thedopant ions away from a purely vertical trajectory so that the scatteredfraction implants into the side walls of the polysilicon gate. (Incontrast, in an ion beam implanter, such scattering is not a feature, sothat only the gate top surface is implanted.) Another advantage of aplasma sheath of finite thickness (and therefore finite collisionalcross-section) is that some very slight scattering of all the ions froma purely vertical trajectory (i.e., a deflection of only a few degrees)may be desirable in some cases to avoid implanting along an axis of thewafer crystal, which could lead to channeling or an implant that is toodeep or a less abrupt junction profile. Also, scattering of the ionsleads to placement of dopants under the polysilicon gate. This can bevery useful in optimizing CMOS device performance by controlling thedopant overlap under the poly Si gate and Source drain extension areas,as will be discussed later in this specification in more detail.

[0292] The low contamination exhibited by the torroidal source PIIIreactor of FIG. 85 is due primarily to the tendency of the plasma to notinteract with chamber surfaces and instead oscillate or circulate in thetorroidal paths that are generally parallel to the chamber surfacesrather than being towards those surfaces. Specifically, the pairtorroidal paths followed by the plasma current are parallel to thesurfaces of the respect reentrant conduits 8150, 8151 of FIG. 85 andparallel to the interior surface of the ceiling 8015 and of the wafersupport pedestal 8025. In contrast, the plasma source power generateselectric fields within the capacitively coupled PIII reactor of FIG. 83Athat are oriented directly toward the ceiling and toward the chamberwalls.

[0293] In the torroidal source PIII reactor of FIG. 85, the onlysignificant electric field oriented directly toward a chamber surface isproduced by the bias voltage applied to the wafer support pedestal 8025,but this electric field does not significantly generate plasma in theembodiment of FIG. 85. While the bias voltage can be a D.C. (or pulsedD.C.) bias voltage, in the embodiment of FIG. 85 the bias voltage is anRF voltage. The frequency of the RF bias voltage can be sufficiently lowso that the plasma sheath at the wafer surface does not participatesignificantly in plasma generation. Thus, plasma generation in thetorroidal source PIII reactor of FIG. 85 produces only plasma currentsthat are generally parallel to the interior chamber surfaces, and thusless likely to interact with chamber surfaces and produce contamination.

[0294] Further reduction of metal contamination of ion implantationprocesses is achieved by first depositing a passivation layer on allchamber surfaces prior to performing the ion implantation process. Thepassivation layer may be a silicon-containing layer such as silicondioxide, silicon nitride, silicon, silicon carbide, silicon hydride,silicon fluoride, boron or phosphorous or arsenic doped silicon, boronor phosphorous or arsenic doped silicon carbide, boron or phosphorous orarsenic doped silicon oxide.

[0295] Alternatively, the passivation may be a fluorocarbon orhydrocarbon or hydrofluorocarbon film. Compounds of germanium may alsobe used for passivation. Alternatively, the passivation layer may be adopant-containing layer such as boron, phosphorous, arsenic or antimonyformed by decomposition of a compound of the dopant precursor gas, suchas BF₃, B₂H₆, PF₃, PF₅, PH₃, AsF₃, of AsH₃. It may be advantageous toform a passivation layer with a source gas or source gas mixture usinggas(es)similar to that or those that are to be used in the subsequentplasma immersion implantation process step. (This may reduce unwantedetching of the passivation layer by the subsequent implant processstep.) Alternatively, it may be advantageous to combine the fluoride andthe hydride of a particular gas to minimize the fluorine and/or hydrogenincorporated in the passivation layer, for example, BF₃+B₂H₆, PH₃+PF₃,AsF₃+AsH₃, SiF₄+SiH₄, or GeF₄+GeH₄.

[0296] While the RF bias frequency of the torroidal source PIII reactorof FIG. 85 is sufficiently low to not affect plasma generation by theplasma source power applicators 8110, 8111, it is also sufficiently lowto permit the ions in the plasma sheath to follow the sheathoscillations and thereby acquire a kinetic energy of up to theequivalent to the full peak-to-peak voltage of the RF bias power appliedto the sheath, depending upon pressure and sheath thickness. Thisreduces the amount of RF bias power required to produce a particular ionenergy or implant depth. On the other hand, the RF bias frequency issufficiently high to avoid significant voltage drops across dielectriclayers on the wafer support pedestal 8025, on chamber interior walls andon the wafer itself. This is particularly important in ion implantationof very shallow junctions, in which the RF bias voltage iscorrespondingly small, such as about 150 volts for a 100 Angstromjunction depth (for example). An RF voltage drop of 50 volts out of atotal of 150 volts across the sheath (for example) would beunacceptable, as this would be a third of the total sheath voltage. TheRF bias frequency is therefore sufficiently high to reduce thecapacitive reactance across dielectric layers so as to limit the voltagedrop across such a layer to less than on the order of 10% of the totalRF bias voltage. A frequency sufficiently high meet this latterrequirement while being sufficiently low for the ions to follow thesheath oscillations is in the range of 100 kHz to 10 MHz, and moreoptimally in the range of 500 kHz to 5 MHz, and most optimally about 2MHz. One advantage of reducing capacitive voltage drops across the waferpedestal is that the sheath voltage can be more accurately estimatedfrom the voltage applied to the pedestal. Such capacitive voltage dropscan be across dielectric layers on the front or back of the wafer, onthe top of the wafer pedestal and (in the case of an electrostaticchuck) the dielectric layer at the top of the chuck.

[0297] Ion implantation results produced by the torroidal source PIIIreactor of FIG. 85 compare favorably with those obtained with aconventional beam implanter operated in drift mode which is much slowerthan the PIII reactor. Referring to FIG. 103, the curves “A” and “a” areplots of dopant (boron) volume concentration in the wafer crystal as afunction of depth for boron equivalent energies of 0.5 keV. (As will bediscussed below, to achieve the same ion energy as the beam implanter,the bias voltage in the PIII reactor must be twice the accelerationvoltage of the beam implanter.) Even though the PIII reactor (curve “A”)is four times faster than the beam implanter (curve “B”), the implantprofile is nearly the same, with the same junction abruptness of about 3nanometers (change in junction depth) per decade (of dopant volumeconcentration) and junction depth (about 100 Angstroms). Curves “B” and“b” compare the PIII reactor results (“B”) with those of a conventionalbeam implanter (“b”) at boron equivalent energies of 2 keV, showing thatthe junction abruptness and the junction depth (about 300 Angstroms) isthe same in both cases. Curves “C” and “c” compare the PIII reactorresults (“C”) with those of a conventional beam implanter (“c”) at boronequivalent energies of 3.5 keV, showing that the junction depth (about500 Angstroms) is the same in both cases.

[0298]FIG. 103 compares the PIII reactor performance with theconventional beam implanter operated in drift mode (in which the beamvoltage corresponds to the desired junction depth).

[0299] Drift mode is very slow because the beam flux is low at such lowbeam energies. This can be addressed by using a much higher beam voltageand then decelerating the beam down to the correct energy before itimpacts the wafer. The deceleration process is not complete, andtherefore leaves an energy “contamination” tail (curve “A” of FIG. 104)which can be reduced by rapid thermal annealing to a better implantprofile with greater abruptness (curve “B” of FIG. 104). Greateractivated implanted dopant concentration, however, can be achieved usinga dynamic surface annealing process employing localized melting ornearly melting temperatures for very short durations. The dynamicsurface annealing process does not reduce energy contamination tails,such as the energy contamination tail of curve “C” of FIG. 105. Incomparison, the torroidal source PIII reactor of FIG. 85 needs nodeceleration process since the bias voltage corresponds to the desiredimplant depth, and therefore has no energy contamination tail (curve “D”of FIG. 105). Therefore, the PIII reactor can be used with the dynamicsurface anneal process to form very abrupt ultra shallow junctionprofile, while the conventional beam implanter operating in decelerationmode cannot. The dynamic surface annealing process consists of locallyheating regions of the wafer surface to nearly (e.g., within 100 to 50degrees of) its melting temperature for very short durations (e.g.,nano-seconds to tens of milliseconds) by scanning a laser beam or agroup of laser beams across the wafer surface.

[0300]FIG. 106 illustrates how much greater a dopant concentration canbe attained with the dynamic surface annealing process. Curve “A” ofFIG. 106 illustrates the wafer resistivity in Ohms per square as afunction of junction depth using a beam implanter and a rapid thermalanneal of the wafer at 1050 degrees C. The concentration of dopantreached 10E20 per cubic centimeter. Curve “B” of FIG. 106 illustratesthe wafer resistivity in Ohms per square as a function of junction depthusing the torroidal source PIII reactor of FIG. 85 and a dynamic surfaceanneal process after implanting at a temperature of 1300 degrees C. Theconcentration of the dopant reached 5×10²⁰ following the dynamic surfaceannealing, or about five times that achieved with rapid thermalannealing. FIG. 107 illustrates how little the implanted dopant profilechanges during dynamic surface annealing. Curve “A” of FIG. 107 is thedopant distribution prior to annealing while curve “B” of FIG. 107 isthe dopant distribution after annealing. The dynamic surface annealingprocess causes the dopant to diffuse less than 10 Å, while it does notadversely affect the junction abruptness, which is less than 3.5nm/decade. This tendency of the dynamic surface annealing process tominimize dopant diffusion facilitates the formation of extremely shallowjunctions. More shallow junctions are required (as source-to-drainchannel lengths are decreased in higher speed devices) in order to avoidsource-to-drain leakage currents. On the other hand, the shallowerjunction require much higher active dopant concentrations (to avoidincreased resistance) that can best be realized with dynamic surfaceannealing. As discussed elsewhere in this specification, junction depthcan be reduced by carrying out a wafer amorphization step in which thewafer is bombarded with ions (such as silicon or germanium ions) tocreate lattice defects in the semiconductor crystal of the wafer. Wehave implanted and annealed junctions having a high dopant concentrationcorresponding to a low resistivity (500 Ohms per square), an extremelyshallow junction depth (185 Å) and a very steep abruptness (less than 4nm/decade). In some cases, the depth of the amorphizing or ionbombardment process may extend below the dopant implant junction depth.For example, amorphization using SiF4 gas and a 10 kV peak-to-peak biasvoltage in the PIII reactor of FIG. 85 forms an amorphized layer to adepth of about 150 Angstroms, while dopant (boron) ions acceleratedacross a 1000 peak-to-peak volt sheath (bias) voltage implant to a depthof only about 100 Angstroms.

[0301]FIG. 108 illustrates the bias voltage for the torroidal sourcePIII reactor (left hand ordinate) and the beam voltage for the ion beamimplanter (right hand ordinate) as a function of junction depth. ThePIII reactor and the beam implanter produce virtually identical resultsprovided the PIII reactor bias voltage is twice the beam voltage.

WORKING EXAMPLES

[0302] A principal application of a PIII reactor is the formation of PNjunctions in semiconductor crystals. FIGS. 109 and 110 illustratedifferent stages in the deposition of dopant impurities in thefabrication of a P-channel metal oxide semiconductor field effecttransistor (MOSFET). Referring first to FIG. 109, a region 9960 of asemiconductor (e.g., silicon) wafer may be doped with an N-typeconductivity impurity, such as arsenic or phosphorus, the region 9960being labeled “n” in the drawing of FIG. 109 to denote its conductivitytype. A very thin silicon dioxide layer 9962 is deposited on the surfaceof the wafer including over n-type region 9960. A polycrystallinesilicon gate 9964 is formed over the thin oxide layer 9962 from ablanket polysilicon layer that has been doped with boron in the PIIIreactor. After formation of the gate 9964, p-type dopant is implanted inthe PIII reactor to form source and drain extensions 9972 and 9973.Spacer layers 9966 of a dielectric material such as silicon dioxideand/or silicon nitride (for example) are formed along two oppositevertical sides 9964 a, 9964 b of the gate 9964. Using the PIII reactorof FIG. 85 with a process gas consisting of BF3 or B2H6 (for example),boron is implanted over the entire N-type region 9960. The spacer layersmask their underlying regions from the boron, so that P-typeconductivity source and drain contact regions 9968, 9969 are formed oneither side of the gate 9964, as shown in FIG. 110. This step is carriedout with a boron-containing species energy in the range of 2 to 10 kvppon the RF bias voltage (controlled by the RF bias power generator 8065of FIG. 85). In accordance with the example of FIG. 108, the RF biasvoltage on the wafer pedestal 8025 in the PIII reactor of FIG. 85 istwice the desired boron energy. The implantation is carried out for asufficient time and at a sufficient ion flux or ion density (controlledby the RF source power generators 8055, 8056 of FIG. 85) to achieve asurface concentration of boron exceeding 5×10¹⁵ atoms per squarecentimeter. The concentration of boron in the gate 9964 is thenincreased to 1×10¹⁶ atoms per square centimeter by masking the sourceand drain contacts 9968, 9969 (by depositing a layer of photoresistthereover, for example) and carrying out a further (supplementary)implantation step of boron until the concentration of boron in the gate9964 reaches the desired level (1×10¹⁶ atoms/cubic centimeter). Thesource and drain contacts 9968, 9969 are not raised to the higher dopantconcentration (as is the gate 9964) because the higher dopantconcentration may be incompatible with formation of a metal silicidelayer (during a later step) over each contact 9968, 9969. However, thegate 9964 must be raised to this higher dopant concentration level inorder to reduce carrier depletion in the gate 9964 near the interfacebetween the gate 9964 and the thin silicon dioxide layer 9962. Suchcarrier depletion in the gate would impede the switching speed of thetransistor. The dopant profile in the gate must be highly abrupt inorder attain a high dopant concentration in the gate 9964 near the thinoxide layer 9962 without implanting dopant into the underlying thinoxide layer 9962 or into the source-to-drain channel underlying the thinoxide layer 9962. Another measure that can be taken to further enhancegate performance and device speed is to raise the dielectric constant ofthe thin silicon dioxide layer 9962 by implanting nitrogen in the thinsilicon dioxide layer 9962 so that (upon annealing) nitrogen atomsreplace oxygen atoms in the layer 9962, as will be described later inthis specification. A further measure for enhancing gate performance isconformal implanting in which dopant ions that have been deflected fromtheir vertical trajectory by collisions in the plasma sheath over thewafer surface are able to implant into the vertical side walls of thegate 9964. This further increases the dopant concentration in the gate9964 near the interface with the thin oxide layer 9962, and provide amore uniform and isotropic dopant distribution within the gate. A yetfurther measure for enhancing gate performance for gates of N-channeldevices implanted with arsenic is to implant phosphorus during thesupplementary implant step using the PIII reactor. The phosphorus islighter than arsenic and so diffuses more readily throughout thesemiconductor crystal, to provide less abrupt junction profile in thesource drain contact areas.

[0303] The depth of the ion implantation of the source and draincontacts 9968, 9969 may be in the range of 400 to 800 Å. If the gate9964 is thinner than that, then the gate 9964 must be implanted in aseparate implantation step to a lesser depth to avoid implanting anydopant in the thin oxide layer 9962 below the gate 9964. In order toavoid depletion in the region of the gate 9964 adjacent the thin oxidelayer 9962, the implantation of the gate must extend as close to thegate/oxide interface as possible without entering the thin oxide layer9962. Therefore, the implant profile of the gate must have the highestpossible abruptness (e.g., 3 nm/decade or less) and a higher dopant dose(i.e., 1×10¹⁶ atoms/cm² ).

[0304] Referring now to FIG. 110, source and drain extensions 9972, 9973are typically formed before depositing and forming the spacer layers9966 of FIG. 109. The extensions layers are formed by carrying out amore shallow and light implant of boron over the entire region 9960.Typically, the junction depth of the source and drain extensions is onlyabout 100 to 300 Angstroms and the implant dose is less than 5×10¹⁵atoms/square centimeter. This implant step, therefore, has little effecton the dopant profiles in the gate 9964 or in the source and draincontacts 9968, 9969, so that these areas need not be masked during theimplantation of the source and drain extensions 9972, 9973. However, ifmasking is desired, then it may be carried out with photoresist. Thesource and drain extensions are implanted at an equivalent boron energyof 0.5 kV, requiring a 1.0 kvpp RF bias voltage on the wafer pedestal8025 of FIG. 85.

[0305] The same structures illustrated in FIGS. 109 and 110 are formedin the fabrication of an N-channel MOSFET. However, the region 9960 isinitially doped with a P-type conductivity such as boron and istherefore a P-type conductivity region. And, the implantation of thegate 9964 and of the source and drain contacts 9968, 9969 (illustratedin FIG. 109) is carried out in a beam implanter (rather than in a PIIIreactor) with an N-type conductivity impurity dopant such as arsenic.Furthermore, the supplementary implantation of the gate 9964 that raisesits dopant dose concentration to 1×10¹⁶ atoms/cm² is carried out in thePIII reactor with phosphorus (rather than arsenic) using aphosphorus-containing process gas. Phosphorus is preferred for thislatter implantation step because it diffuses more uniformly thanarsenic, and therefore enhances the quality of the N-type dopant profilein the gates 9964 of the N-channel devices. The ion beam voltage is inthe range of 15-30 kV for the arsenic implant step (simultaneousimplanting of the N-channel source and drain contacts 9968, 9969 and ofthe N-channel gates 9964), and is applied for a sufficient time to reacha dopant surface concentration exceeding 5×10¹⁵ atoms per cubiccentimeter. The supplementary gate implant of phosphorus is carried outat an ion beam voltage in the range of only 2-5 kV for a sufficient timeto raise the dopant surface concentration in the N-channel gates to1×10¹⁶ atoms/cubic cm.

[0306] The implantation steps involving phosphorus and boron areadvantageously carried out in the PIII reactor rather than an ion beamimplanter because the ion energies of these light elements are so lowthat ion flux in a beam implanter would be very low and the implanttimes would be inordinately high (e.g., half and hour per wafer). In thePIII reactor, the source power can be 800 Watts at 13.56 MHz (with the200 kHz offset between the two torroidal plasma currents as describedabove), the implant step being carried out for only 5 to 40 seconds perwafer.

[0307] The sequence of ion implantation steps depicted in FIGS. 109 and110 may be modified, in that the light shallow source and drainextension implant step of FIG. 110 may be carried out before or afterformation of the spacer layer 9966 and subsequent heavy implantation ofthe contacts 9968, 9969 and gate 9964. When extension implants are doneafter the spacer layer 9966 is formed, the spacer layer 9966 must beremoved before the extension implants are performed.

[0308] One example of a process for fabricating complementary MOSFETS(CMOS FETs) is illustrated in FIG. 111. In the first step (block 9980),the P-well and N-well regions of the CMOS device are implanted inseparate steps. Then, a blanket thin gate oxide layer and an overlyingblanket polysilicon gate layer are formed over the entire wafer (block9981 of FIG. 111). The P-well regions are masked and the N-well regionsare left exposed (block 9982). The portions of the polysilicon gatelayer lying in the N-well regions are then implanted with boron in aPIII reactor (block 9983). The P-channel gates (9964 in FIG. 109) arethen photolithographically defined and etched, to expose portions of thesilicon wafer (block 9984). Source and drain extensions 9972, 9973 ofFIG. 109 self-aligned with the gate 9964 are then formed by ionimplantation of boron using the PIII reactor (block 9985). A so-called“halo” implant step is then performed to implant an N-type dopant underthe edges of each P-channel gate 9964 (block 9986). This is done byimplanting arsenic using an ion beam tilted at about 30 degrees from avertical direction relative to the wafer surface and rotating the wafer.Alternatively, this step may be accomplished by implanting phosphorus inthe PIII reactor using a chamber pressure and bias voltage conducive toa large sheath thickness to promote collisions in the sheath that divertthe boron ions from a vertical trajectory. Then, the spacer layers 9986are formed over the source drain extensions 9972, 9973 (block 9987) andboron is then implanted at a higher energy to form the deep source draincontacts 9969 (block 9988), resulting in the structure of FIG. 110. Thereverse of step 9982 is then performed by masking the N-well regions(i.e., the P-channel devices) and exposing the P-well regions (block9992). Thereafter steps 9993 through 9998 are performed that correspondto steps 9983 through 9988 that have already been described, except thatthey are carried out in the P-well regions rather than in the N-wellregions, the dopant is Arsenic rather than Boron, and a beam line ionimplanter is employed rather than a PIII reactor. And, for the N-channeldevice halo implant of block 9996 (corresponding to the P-channel devicehalo implant of block 9986 described above), the dopant is a P-typedopant such as boron. In the case of the N-channel devices implanted insteps 9993 through 9998, a further implant step is performed, namely asupplemental implant step (block 9999) to increase the dose in thepolysilicon gate as discussed above in this specification. In thesupplemental implantation step of block 9999, phosphorus is the N-typedopant impurity and is implanted using a PIII reactor rather than a beamimplanter (although a beam implanter could be employed instead).

[0309] As noted above, the process may be reversed so that the gate 9964and source and drain contacts 9968, 9969 are implanted before the sourceand drain extensions 9972, 9973.

[0310] After all ion implantations have been carried out, the wafer issubjected to an annealing process such as spike annealing using rapidthermal processing (RTP) and/or the dynamic surface annealing (DSA)process discussed earlier in this specification. Such an annealingprocess causes the implanted dopant ions, most of which came to rest ininterstitial locations in the crystal lattice, to move to atomic sites,i.e., be substituted for silicon atoms originally occupying those sites.More than one annealing step can be used to form the pmos and nmosdevices and these steps can be inserted in the process flow asappropriate from activation and diffusion point of view.

[0311] The foregoing ion implantation processes involving the lighteratomic elements (e.g., boron and phosphorus) are carried out using aPIII reactor in the modes described previously. For example, the biaspower frequency is selected to maximize ion energy while simultaneouslyproviding low impedance coupling across dielectric layers. How this isaccomplished is described above in this specification.

[0312] The ion implantation processes described above are enhanced byother processes. Specifically, in order to prevent channeling and inorder to enhance the fraction of implanted ions that becomesubstitutional upon annealing, the semiconductor wafer crystal may besubjected to an ion bombardment process that partially amorphizes thecrystal by creating crystal defects. The ions employed should becompatible with the wafer material, and may be formed in the PIIIreactor in a plasma generated from one or more of the following gases:silicon fluoride, silicon hydride, germanium fluoride, germaniumhydride, Xenon, Argon, or carbon fluoride (ie. tetrafluoromethane,octafluorocyclobutane, etc) or carbon hydride (ie. methane, acetylene,etc) or carbon hydride/fluoride (ie. tetrafluoroethane,difluoroethylene, etc) gases. One advantage of the PIII reactor is thatits implant processes are not mass selective (unlike an ion beamimplanter). Therefore, during ion implantation of a dopant impurity suchboron, any other element may also be implanted simultaneously,regardless of ion mass in the PIII reactor. Therefore, unlike an ionbeam implanter, the PIII reactor is capable of simultaneously implantinga dopant impurity while carrying out an amorphizing process. This may beaccomplished using a BF3 gas (to provide the dopant ions) mixed with anSiF4 gas (to provided the amorphizing bombardment ion species). Such asimultaneous ion implantation process is referred to as a co-implantprocess. The amorphization process may also be carried out sequentiallywith the doping process. In addition to amorphization, simultaneousimplants of dopant and non-dopant atoms such as Fluorine, Germanium,Carbon or other elements are done to change the chemistry of the Siliconwafer. This change in chemistry can help in increasing dopant activationand reducing dopant diffusion.

[0313] Another process that can be carried out in the PIII reactor is asurface enhancement process in which certain ions are implanted in orderto replace other elements in the crystal. One example of such a surfaceenhancement process is nitrodization. In this process, the dielectricconstant of the thin silicon dioxide layer 9962 is increased (in orderto increase device speed) by replacing a significant fraction of theoxygen atoms in the silicon dioxide film with nitrogen atoms. This isaccomplished in the PIII reactor by generating a plasma from anitrogen-containing gas, such as ammonia, and implanting the nitrogenatoms into the silicon dioxide layer 9962. This step may be performed atany time, including before, during and/or after the implantation of thedopant impurity species. If the nitrodization process is performed atleast partially simultaneously with the dopant ion implant step, thenthe nitrodization process is a co-implant process. Since the ionimplantation process of the PIII reactor is not mass selective, theco-implant process may be carried out with any suitable species withoutrequiring that it atomic weight be the same as or related to the atomicweight of the dopant implant species. Thus, for example, the dopantspecies, boron, and the surface enhancement species, nitrogen, havequite different atomic weights, and yet they are implantedsimultaneously in the PIII reactor. Typically nitrodization is performedwithout implanting dopant atoms.

[0314] A further process related to ion implantation is surfacepassivation. In this process, the interior surfaces of the reactorchamber, including the walls and ceiling, are coated with asilicon-containing passivation material (such as silicon dioxide orsilicon nitride or silicon hydride) prior to the introduction of aproduction wafer. The passivation layer prevents the plasma frominteracting with or sputtering any metal surfaces within the plasmareactor. The deposition the passivation layer is carried out by ignitinga plasma within the reactor from a silicon containing gas such as silanemixed with oxygen, for example. This passivation step, combined with thelow-contamination torroidal source PIII reactor of FIG. 85, has yieldedextremely low metal contamination of a silicon wafer during ionimplantation, about 100 times lower than that typically obtained in aconventional beam implanter.

[0315] Upon completion of the ion implantation process, the passivationlayer is removed, using a suitable etchant gas such as NF3 which may becombined with a suitable ion bombardment gas source such as argonoxygen, or hydrogen. During this cleaning step, the chamber surfaces maybe heated to 60 degrees C. or higher to enhance the cleaning process. Anew passivation layer is deposited before the next ion implantationstep.

[0316] Alternatively, a new passivation layer may be deposited beforeimplanting a sequence of wafers, and following the processing of thesequence, the passivation layer and other depositions may be removedusing a cleaning gas.

[0317]FIG. 112 is a flow diagram showing the different options ofcombining the foregoing ion implantation-related processes with thedopant implantation processes of FIG. 111. A first step is cleaning thechamber to remove contamination or to remove a previously depositedpassivation layer (block 9001 of FIG. 112). Next, a passivation layer ofsilicon dioxide, for example, is deposited over the interior surfaces ofthe chamber (block 9002) prior to the introduction of the wafer to beprocessed. Next, the wafer is introduced into the PIII reactor chamberand may be subjected to a cleaning or etching process to remove thinoxidation layers that may have accumulated on the exposed semiconductorsurfaces in the brief interim since the wafer was last processed (block9003). A pre-implant wafer amorphizing process may be carried out (block9004) by ion-bombarding exposed surfaces of the wafer with silicon ions,for example. A pre-implant surface enhancement process may also becarried out (block 9005) by implanting a species such as nitrogen intosilicon dioxide films. The dopant implantation process may then becarried out (block 9006). This step is an individual one of the boron orphosphorus implant steps illustrated in the general process flow diagramof FIG. 111. During the dopant implant process of block 9006, other ionsin addition to the dopant ions may be implanted simultaneously in aco-implant process (block 9007). Such a co-implant process (9007) may bean amorphizing process, a light etch process that prevents accumulationof plasma by-products on the wafer surface, enhancing dopant activationand reducing dopant diffusion, or surface enhancement process. Aftercompletion of the dopant ion implant process (9006) and any co-implantprocess (9007), various post implant processes may be carried out. Suchpost implant processes may include a surface enhancement process (block9008). Upon completion of all implant steps (including the step of block9008), an implant anneal process is carried out (block 9012) afterremoving any photo-resist mask layers on the wafer in the precedingwafer clean step of block 9009. This anneal process can be a dynamicsurface anneal in which a laser beam (or several laser beams) arescanned across the wafer surface to locally heat the surface to nearlymelting temperature (about 1300 degrees C.) or to melting temperature,each local area being heated for an extremely short period of time(e.g., on the order of nanoseconds to tens of milliseconds). Other postimplant processes carried out after the anneal step of block 9112 mayinclude a wafer cleaning process (block 9009) to remove layers of plasmaby-products deposited during the ion implantation process, deposition ofa temporary passivation coating on the wafer to stabilize the wafersurface (block 9010) and a chamber cleaning process (block 9011),carried out after removal of the wafer from the PIII reactor chamber,for removing a previously deposited passivation layer from the chamberinterior surfaces.

[0318] While the invention has been described in detail by specificreference to preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A plasma immersion ion implantation reactor forion implanting a species into a surface layer of a workpiece,comprising: an enclosure comprising a side wall and a ceiling anddefining a chamber; a workpiece support pedestal within the chamberhaving a workpiece support surface facing said ceiling and defining aprocess region extending generally across said wafer support pedestaland confined laterally by said side wall and axially between saidworkpiece support pedestal and said ceiling; said enclosure having atleast a first pair of openings at generally opposite sides of saidprocess region; a first hollow conduit outside of said chamber havingfirst and second ends connected to respective ones of said first pair ofopenings, so as to provide a first reentrant path extending through saidconduit and across said process region; gas distribution apparatus on ornear an interior surface of said reactor for introducing a process gascontaining the species to be ion implanted; a first RF plasma sourcepower applicator for generating a plasma in said chamber.
 2. The reactorof claim 1 wherein said enclosure further comprises a base, and saidinterior surface of said reactor comprises one of: (a) said ceiling, (b)said side wall, (c) said conduit, (d) said base.
 3. The reactor of claim1 further comprising a pumping annular volume defined between saidworkpiece support pedestal and said side wall, and a vacuum pump coupledto said pumping annular volume.
 4. The reactor of claim 1 wherein saidfirst RF source power applicator is positioned to couple RF plasmasource power into a region of said first hollow conduit.
 5. The reactorof claim 1 wherein said plasma comprises a plasma current in saidreentrant path that oscillates at an RF frequency of said first RFplasma source power applicator.
 6. The reactor of claim 1 wherein saidfirst hollow conduit comprises a metal material, said reactor furthercomprising: an annular insulating gap in said first hollow conduitseparating said first hollow conduit into axial sections.
 7. The reactorof claim 5 wherein said ceiling comprises a constriction of saidreentrant torroidal path in said process zone for enhancement of plasmaion density of said plasma current in said process zone.
 8. The reactorof claim 5 wherein said ceiling and said wafer support pedestal areseparated by a gap therebetween, said gap being sufficiently small sothat plasma ion density of said plasma current is greater in thevicinity of said workpiece support pedestal than elsewhere along saidrentrant path.
 9. The reactor of claim 1 wherein the workpiece supportpedestal comprises an electrostatic chuck, said electrostatic chuckcomprising thermal control apparatus for workpiece temperature control.10. The reactor of claim 9 further comprising a bias RF power generatorcoupled to said workpiece support pedestal to control a sheath voltageacross a plasma sheath overlying said workpiece support pedestal. 11.The reactor of claim 9 further comprising a bias RF voltage generatorcoupled to said workpiece support pedestal to control a sheath voltageacross a plasma sheath overlying said workpiece support pedestal. 12.The reactor of claim 1 further comprising a gas supply containing saidprocess gas.
 13. The reactor of claim 12 wherein said species to beimplanted comprises a first atomic element, said process gas furthercomprising: a second atomic element in chemical combination with saidfirst atomic element.
 14. The reactor of claim 13 wherein said surfacelayer of said workpiece is a semiconductor material and said firstatomic element is an n-type or p-type conductivity dopant impurity withrespect to said semiconductor material.
 15. The reactor of claim 14wherein said second atomic element comprises a semiconductor element.16. The reactor of claim 15 wherein said second atomic element and saidsemiconductor material of said surface layer are the same atomicelement.
 17. The reactor of claim 14 wherein said second atomic elementis an element having a greater tendency than said first atomic elementfollowing ion implantation to diffuse out of said surface layer uponheating of said surface layer.
 18. The reactor of claim 14 wherein saidsecond atomic element comprises one of hydrogen and fluorine.
 19. Thereactor of claim 14 wherein the chemical combination of said first andsecond atomic species comprises a first molecular species, said processgas further comprising a second molecular species.
 20. The reactor ofclaim 19 wherein said second molecular species comprises one of: (a)hydrogen-containing gas, (b) fluorine-containing gas.
 21. The reactor ofclaim 19 wherein said second molecular species comprises a diluent gas.22. The reactor of claim 21 wherein said first molecular speciescomprises a fluoride of said dopant impurity and said second molecularspecies comprises a hydride of said dopant impurity.
 23. The reactor ofclaim 22 wherein said process gas further comprises a third molecularspecies.
 24. The reactor of claim 23 wherein said third molecularspecies comprises a diluent gas.
 25. The reactor of claim 23 whereinsaid third molecular species comprises at least one of (a)hydrogen-containing gas, (b) fluorine-containing gas, (c) an inert gas.26. The reactor of claim 1 further comprising: a second pair of openingsthrough said enclosure at generally opposite sides of said processregion and displaced from said first pair of openings; a second hollowconduit outside of said chamber having first and second ends connectedto respective ones of said second pair of openings, so as to provide asecond reentrant path through said conduit and across said processregion transverse to said first reentrant path; and a second plasmasource power applicator coupled to said second hollow conduit forgenerating a plasma current in said second reentrant path.
 27. Thereactor of claim 26 further comprising one or more RF power source forfurnishing RF power to said first and second RF power applicators. 28.The reactor of claim 27 wherein said RF power source furnishes RF powerto said first and second RF power applicators at respective RFfrequencies offset from one another.
 29. The reactor of claim 26 whereinsaid first and second paths are non-parallel and intersect one anotherin said process region.
 30. The reactor of claim 29 wherein said firstand second paths are generally orthogonal to one another.
 31. Thereactor of claim 1 wherein said gas distribution apparatus comprise agas distribution plate on said ceiling.
 32. The reactor of claim 1wherein said gas distribution apparatus comprises a gas distributionring on said wall.
 33. The reactor of claim 1 wherein said enclosurefurther comprises a base, and said gas distribution apparatus comprisesa plurality of discrete gas injection nozzles or diffusers on one of:(a) said side wall, (b) said ceiling, (c) base of said chamber.
 34. Thereactor of claim 26 wherein said first and second RF power applicatorscomprise respective conductors coiled around respective ones of saidfirst and second hollow conduits, said respective conductors beingcoupled to respective ones of said first and second RF power sources.35. The reactor of claim 26 wherein said ports are in said ceiling,whereby said hollow conduits terminate axially into said ports.
 36. Thereactor of claim 26 wherein said ports are in said side wall, wherebysaid hollow conduits terminate radially into said ports.
 37. The reactorof claim 26 wherein said ports are in said base of said chamber, wherebysaid hollow conduits terminate axially into said ports.
 38. The reactorof claim 1 further comprising a bias power generator coupled to saidworkpiece support pedestal to control a sheath voltage across a plasmasheath overlying said workpiece support pedestal.
 39. The reactor ofclaim 1 further comprising a bias voltage generator coupled to saidworkpiece support pedestal to control a sheath voltage across a plasmasheath overlying said workpiece support pedestal.
 40. The reactor ofclaim 38 wherein said bias power generator has an RF bias frequencysufficiently low to enable ions traversing the plasma sheath to attainan energy corresponding to a peak-to-peak voltage of said bias powergenerator.
 41. The reactor of claim 38 wherein said bias power generatorhas an RF frequency sufficiently high to limit RF voltage drops acrossdielectric layers on said workpiece support pedestal to less than apredeterminded fraction of plasma sheath voltage near said workpiecesupport.
 42. The reactor of claim 41 wherein said predetermined fractioncorresponds to about 10%.
 43. The reactor of claim 28 wherein saidoffset frequencies are centered around 10 MHz and are offset by about200 kHz.
 44. The reactor of claim 26 wherein said first and secondhollow conduits are non-intersecting whereby said first and second pathsintersect one another only in said process region.
 45. The reactor ofclaim 1 wherein said gas distribution apparatus is in said ceiling andcomprises a center orifice and plural outer orifices in a circlecentered on said center orifice, said reactor further comprising: a gaspanel containing a separate gas supplies for respective process gasesfor doping and for passivating and for removing; and a gas distributioncontroller comprising a first set of valves coupling at least one ofsaid separate gas supplies to said center orifice and a second set ofvalves coupling at least some of said separate gas supplies to saidplural outer orifices.
 46. The reactor of claim 1 wherein said gasdistribution apparatus comprises first and second sets of pluralorifices, said reactor further comprising: a gas panel containing aseparate gas supplies for respective process gases for doping and forpassivating and for removing; and a gas distribution controllercomprising a first set of valves coupling at least one of said separategas supplies to said first set of plural orifices and a second set ofvalves coupling at least some of said separate gas supplies to saidsecond set of plural orifices.
 47. The reactor of claim 45 wherein: saidgases for doping comprise a fluoride of a dopant species and a hydrideof a dopant species, said gases for passivating comprise a hydride of apassivating species and a fluoride of a passivating species, said gasesfor removing comprise an etchant-containing gas and an inert gas; andsaid gases for oxidizing comprise oxygen.
 48. The reactor of claim 46wherein: said gases for doping comprise a fluoride of a dopant speciesand a hydride of a dopant species, said gases for passivating comprise ahydride of a passivating species and a fluoride of a passivatingspecies, said gases for removing comprise an etchant-containing gas andan inert gas; and said gases for oxidizing comprise oxygen.
 49. Thereactor of claim 47 wherein said gas distribution controller furnishesoxygen exclusively to said center orifice.
 50. The reactor of claim 48wherein said gas distribution controller furnishes oxygen exclusively tosaid second set of plural orifices.
 51. The reactor of claim 38 furthercomprising a controller for controlling said bias power generator toproduce a desired bias voltage at said workpiece support pedestal for apredetermined single burst duration.
 52. The reactor of claim 51 whereinsaid controller comprises: a timer for switching the output of said biaspower generator on and off in accordance with said predeterminedduration; a peak voltage detector coupled to said workpiece supportpedestal; a threshold comparator connected to said timer for comparingthe output of said peak voltage detector with a predetermined thresholdvoltage; a subtractor having a pair of inputs connected to the output ofsaid peak voltage detector and to a predetermined target voltage,respectively, and a feedback conditioner for processing the output ofsaid subtractor; a first switch for coupling an output of said feedbackconditioner to a power level control input of said bias power generator;53. The reactor of claim 52 wherein said controller further comprises acontrol element for controlling said bias power generator (a)empirically in absence of a plasma in said chamber and (b) in a feedbackcontrol loop in the presence of plasma in said chamber.
 54. The reactorof claim 53 wherein said control element comprises: a voltage-to-powerlook-up table having an input connected to said predetermined targetvoltage and an output; a second switch coupled between the output ofsaid voltage-to-power look-up table and said power level control inputof said bias power generator; and a plasma detector in said chamberconnected to control said first and second switches in complementaryfashion in response to detection of plasma in said chamber.
 55. Thereactor of claim 54 wherein said plasma detector is further connected toenable said timer.
 56. The reactor of claim 52 wherein said feedbackconditioner is an integral proportional controller.
 57. The reactor ofclaim 52 wherein said predetermined threshold voltage and saidpredetermined target voltage are identical.
 58. The reactor of claim 52further comprising a process controller for furnishing saidpredeterminded target voltage and said predetermined threshold voltage.59. The reactor of claim 1 further comprising a vacuum pump and a vacuumcontrol valve coupling said vacuum pump to said chamber, said vacuumcontrol valve comprising: a valve housing having a valve opening definedby an opening side wall having a surface parallel to an axis of saidvalve opening; a rotatable flap subject to process control and having anarea conformal with said valve opening and side wall and rotatablymounted within said valve opening to define a gap therebetween; and aplurality of small indentational voids in said side wall that arecovered by said rotatable flap whenever said flap is in a co-planarrelationship with said housing and are gradually exposed as said flaprotates away from said rotational position and before a bottom corneredge of said flap passes a top surface of said valve housing.
 60. Thereactor of claim 1 wherein said workpiece support pedestal comprises: aconductive wafer support plate; a grounded conductive base plate formingat least a void between said support and base plates; a side wall aroundsaid support and base plates forming at least a void between said sidewall and said support and base plates; a high dielectric filler materialhaving a high break-down voltage filling said voids; and a conductiveinsert coupled to said bias power generator and a conductive femalereceptacle for tightly receiving said conductive insert, said conductivefemale receptable being connected to said conductive wafer supportplate, said conductive insert and said conductive female receptableextending through said conductive base plate to said conductive wafersupport plate, and insulating layer insulating said conductive insertfrom said conductive base plate.
 61. The reactor of claim 60 whereinsaid workpiece support pedestal further comprises at least one lift pinassembly extending through said conductive base plate and saidconductive wafer support plate and a axial void between said lift pinassembly and said lift pin assembly, and a high dielectric fillermaterial having a high breakdown voltage within the void between saidlift pin assembly and said conductive wafer support plate.
 62. Thereactor of claim 61 further comprising a fastening bolt extending atleast partially through said conductive wafer support plate and to saidconductive base plate, and a high dielectric filler material having ahigh breakdown voltage surrounding a portion of said bolt within saidconductive wafer support plate.
 63. The apparatus of claim 1 furthercomprising an RF source power generator coupled to said first RF powerapplicator and an RF bias power generator coupled to said workpiecesupport pedestal.
 64. The apparatus of claim 63 wherein said RF sourcepower generator and said RF bias power generator comprise first andsecond pulsed RF supplies, respectively.
 65. The apparatus of claim 64wherein said first and second pulsed RF supplies are in a push-pullrelationship.
 66. The apparatus of claim 64 wherein said first andsecond pulsed RF supplies are in an in-synchronism relationship.
 67. Theapparatus of claim 64 wherein said first and second RF supplies are in asymmetric relationship.
 68. The apparatus of claim 64 wherein said firstand second RF supplies are in a non-symmetric relationship.
 69. Theapparatus of claim 1 further comprising: an RF source power generatorconnected to said first RF power applicator; a D.C. bias power generatorconnected to said wafer support pedestal.
 70. The apparatus of claim 69wherein: said RF source power generator and said D.C. bias powergenerator supply comprise first and second pulsed power supplies. 71.The apparatus of claim 70 wherein said first and second pulsed powersupplies are in a push-pull relationship.
 72. The apparatus of claim 70wherein said first and second pulsed power supplies are in anin-synchronism relationship.
 73. The apparatus of claim 70 wherein saidfirst and second pulsed power supplies are in a symmetricalrelationship.
 74. The apparatus of claim 70 wherein said first andsecond pulsed power supplies are in a non-symmetrical relationship. 75.The apparatus of claim 38 wherein said bias power generator has a biasfrequency that is sufficiently low for ions in a plasma sheath near saidworkpiece to follow electric field oscillations across said sheath atsaid bias frequency.
 76. The apparatus of claim 75 wherein said biasfrequency is sufficiently high so that RF voltage drops acrossdielectric layers on said workpiece do not exceed a predeterminedfraction of the RF bias voltage applied to said workpiece support. 77.The apparatus of claim 76 wherein said predetermined fractioncorresponds to about 10%.
 78. The apparatus of claim 38 wherein saidbias power generator has a bias frequency between 10 kHz and 10 MHz. 79.The apparatus of claim 38 wherein said bias power generator has a biasfrequency between 50 kHz and 5 MHz.
 80. The apparatus of claim 38wherein said bias power generator has a bias frequency between 100 kHzand 3 MHz.
 81. The apparatus of claim 38 wherein said bias powergenerator has a bias frequency of about 2 MHz to within about 5%.